best advice for the interior/exterior is to browse, browse, browse. Find lots of journals and build threads and see what you like.
Some ideas to get you started
Exterior:
Wheels (with a lower effective offset?)
Lip/Bumper
Fog Lights
Wing (Aftermarket or wingless)
Sideskirts
Diffuser
Tint
Rain Guards
Lowered (requires springs or coilovers)
Grill
Interior:
Seats (looks and performance!)
Gauges
Steering wheel
Stereo stuff
Custom Upholstery/Headliner
LED Interior Lights
Custom Lighting
"Red"ing the interior (change all lights, like HVAC controls, to be red not green)
Rollcage
Roof Vent
As for power, we've gotta know your goals and budget!