I can't believe we don't have this thread yet! I'm going to do the basics and as more of you guys check it out, you can let me know what I missed out on!
Before you ever purchase a used vehicle, there are some things you need to look for to make sure the vehicle if in full, proper running condition.
1) Show me the CarFax! Every vehicle that you'll ever be purchasing has to have a CarFax, that is a given. Make sure the vehicle didn't have any major accidents where an important component of the vehicle was damaged. A good example of this is an accident where the frame of the car was damaged. This is a safety issue and you should avoid it at all costs. If the vehicle was in a minor accident, and there are receipts for proper, professional repairs, then this can sometimes work as a plus on your side since the vehicle would be going for a lower rate than "KBB." Unfortunately, every dealer and private seller goes by the blue book, this is just the life we live in. Even though it's not 100% correct, this is pretty much one way of seeing how much the specific year or model is currently going for. The CarFax will tell you every accident the car was in (if the accident was reported to the insurance agency), all of the -reported- maintenance done on the car, how many owners the car had, for how long each owner owned the vehicle, and some other information. (I can't remember it all)
2) Check to see how many miles the vehicle has. Make sure the mileage on the odometer matches the mileage on the CarFax. A mod that is available for these cars is the gauge cluster swap from a different model. (Usually STi clusters in WRX's and sometimes Prodrive clusters in STi's) Mileage is a big factor when it comes to these cars because the higher the mileage on the car, the higher the chance the car could have not been taken care of the car was abused. The cars that have a single owner and that can provide a history on the vehicle, usually maintained it well. There are very picky owners out there that must do oil changes every 2,500 or 3,000 miles with a specific brand and type of oil. (I would be one of those people ) Also with mileage, when you get to the higher numbers, make sure the timing belt was changed along with the water pump. There is a kit for our cars that replaces all of the important components when we need to change the timing belt.
3) Get familiar with what a stock WRX or STi engine bay should look like. Once you pop that hood and see all the fancy parts, I'm certain your breath will be taken away by the awesomeness. Unfortunately, this could be a potential threat to the car if it has parts under the hood and the vehicle wasn't tuned for it. Common parts under the hood that are swapped are: pulleys, short ram intakes, cold air intakes, top mount intercooler (the car could also have a front mount intercooler), turbo, and by pass valve (could also have a blow off valve). If any of these components are not OEM, then there is a possibility the owner did not get the car tuned. Always ask if the car was tuned. If you're buying from a dealer and you ask them about the car being tuned, you could be lied to, so ask for proof!!!
4) Check the rims and tires! Make sure the vehicle has OEM rims and good branded tires with good tread. If the vehicle doesn't have the stock rims, then make sure the rims are of equivalent or more value than the stock rims. The dealer could be hustling you for more money with some cheap, off brand rims! A VERY IMPORTANT THING - make sure all the tires are within 2/32nd tread of each other. Generally speaking, if the rims are swapped every oil change, then the wear should be even! (Example: all 4 tires have 7/32nd tread) Here's an example of what must be looked up and avoided if found: If the tread on the front 2 tires are 7/32nd and the rear 2 tires are 3/32nd, move on to the next car because this will ruin the drivetrain! Our cars must have all tires within 2/32nd of each other so the front or rear differentials don't spin less or more than the other. This will cause severe damage to the drivetrain in return.
5) Check the suspension and make sure the vehicle has either stock suspension or a good brand named coilovers. If the vehicle has cheap, offbrand coilovers, then there is a chance they could be blown or are waiting to go out on your drive home. There are other threads on here that tell you which coilovers are good and which ones to avoid. Also, when looking under the vehicle, check to make sure there are no other suspension parts that are aftermarket. Examples of such parts are: sway bars, end links, control arms, bushings, subframes, drive shaft, strut bars, and h-braces. If there are aftermarket parts, make sure they were installed correctly and are loose.
6) Brakes and brake pads. Make sure the vehicle has OEM calipers and brake pads with a good amount of life left. If the vehicle doesn't have the OEM calipers, then make sure the ones on there are of equivalent or more value. (Some people could swap the Brembos for 6 piston calipers, that is NOT a bad thing for you - only for your wallet! ) If the car has brake pads with little to no life left, make sure that is brought to the dealer's attention.
7) Are the body panels OEM or aftermarket? Some people like their carbon fiber more than their aluminium. I don't blame them! Common parts that are swapped for cabon fiber ones are the hood, the trunk, the spoiler, the mirrors, the front fenders, and the hood scoop. Some people take it over the top and get front bumpers, rear bumpers, doors, door panels, and roof. Carbon fiber is lighter, however in some cases, it is heavier than the stock component. If you're going to be daily driving your car, you don't need carbon fiber since you're going to be dealing with a lot of rock chips! Also when you're looking at the body panels, one thing you can do is look for different paint coloration and make sure the panels align with each other. If something was painted recently, you'll definitely be able to tell. If the panels don't align, you know the vehicle was damaged at some point.
8) Moving on to the interior! The most popular modifications for the interior are: seats, center console, shift knob, head unit, gauges, gauge pods, and steering wheel. Again, get familiar with what the stock model looks like before checking any vehicles out. This will give you a good idea for what to look out for when you're finally inspecting. Always check the trunk of the car. Make sure the original spare tire is in there along with the tools and jack.
9) Ask the dealer or private seller to get a compression and leakdown test performed on the vehicle. The results will tell you if there is anything wrong with the engine or if it's in a healthy condition. If any dealer or private seller refuses to get these tests done, smile and say "Thank you for your time and have a great day!" The only reason these tests would be refused is it there is something wrong with the engine and they don't want you knowing.
10) ALWAYS test drive the car before you purchase! That is a given! This way you can see how the car reacts to normal driving, aggressive acceleration, wide open throttle if permitted by the private seller (dealers don't care), handling, braking, cluth grab and stiffness, gear shifting (could have a short throw shifter), heat works, air conditioning works, power doors work, power windows work, power mirrors work, and stability on normal roads and the highway.
11) If purchasing a vehicle that has aftermarket parts, always ask if the OEM parts are included with the purchase. Always ask if the car has an accessport that comes with it and if it doesn't, make sure it has the stock intake and turbo back exhaust on it. (There are vehicles out there that are married to accessports and you don't receive it in the deal -GREAT CAUSE FOR CONCERN!!!!)
12) Every state has different emissions and laws regarding a vehicle. Make sure the exhaust isn't too loud if you have a noise ordinance in your area, the tint isn't too dark so the police don't hand out easy tickets, and that there is a catalytic converter if your area has pollution control.
13) Always have a second person with greater knowledge available to help aid you in purchasing the vehicle if you've come this far in the checklist! There could be something you overlooked that the other person caught!
14) If the car is stock, then it will have a stock, factory tune on it. (There are those rare occurrences where someone kept the car completely stock, but used an accessport to make it stage 1) If there are any parts on the car that "suck and blow" and are not from factory, make sure it has an accessport because these are parts that REQUIRE a tune in order for your car to function properly. The accessport has OTS maps (off the shelf) that can be installed on the vehicle if you have the correct parts. Example: Stage 1 doesn't require anything on the car to be modified, it's just a better tune that the factory one. Stage 2, however, requires an aftermarket down pipe and catback WITH the STOCK intake. If the vehicle doesn't have a stock intake, then a protune must have been done or the car will not function properly. The only exceptions to this are the Cobb short ram intake and the AEM short ram intake for certain Stage 2 OTS maps.
I'm sure I missed A LOT of things, so fellas, help me expand this and correct it if need be! I'll be making it more fancy at a later point, but I just spent an hour typing all of this out and I need a break!