Lots of ideas, but let's break it down into three categories:
1) Power
2) Handling
3) Cosmetic
Power
The problem with doing power is that you end up spending lots of money if you do things incrementally. For example, getting a new downpipe now, but then down the road you decide to go rotated, you have to get yet another downpipe. Plus pretty much each time you mod for power, you need to get a tune done, which can cost a few hundred. The benefit is Power = Fun! Oh yeah, and warranty issues on a new car. Some dealers are cool with it (especially if they sell you the parts...usually at full MSRP)
Handling
Handling is lots of fun. The only real "problem" here is that sometime you make the car harsher over normal roads as a sacrifice for more grip. Not everything you do will have that effect (eg Swaybars) but lots of things will/can (bushings, springs, etc...). The benefit is that, for the most part, you can do it piece by piece year after year to get you exactly where you want to be. There's none of that pesky tuning business like for power, so you dont feel compelled to stockpile parts and install them all at once. In fact, with suspension bits, its often fun to do them one at a time, so you can see the benefits that each part makes.
Cosmetic
This one is the first thing some people do, and the last that others do. No real gains other than potentially liking your car more. That being said, it really can make a difference. I did a few subtle things that I think really make my car look better and were relatively cheap. Dont discount cosmetic just because you wont be able to "feel" a difference.
Ok, all that being said--whats your long term plan for the car? You said you had a vision, you should start a journal and post it up. That'll really help us make suggestions.
Also check out this thread...lots of good links:
http://www.igotasti.com/vBforum/showthread.php?4702-Engine-Threads-of-Interest