My 2c for what it's worth:
How about keeping it stock, or very slightly modified? I had an e30 325i at school and an e36 325i at college and in my opinion I over-did it. 17" rims, strut braces, Koni adjustables, full exhaust, airbox, cams, intakes, body kits, and more. At the time I thought they looked cool and were seriously quick but (after a couple of years) I came back down to earth. *It's a BMW 3 series*. After I sold the E36 I bought a 993 and realised that I'd wasted a lot of time and money. The BMs I modded were kind of fun on an open road but terrible in town or traffic - I'd compromised the package trying to build against what the BM engineers had designed. They'd put a lot of work into those two 325i models and when it came down to it I was trying to build an M3 on the cheap - it would've been better to have bought an M3 and been done with it.
It's easy to get carried away, hence the existence of a huge number of so-called 'tuners'...
Also, it's worth mentioning that those mods added a premium of about 5% to the book price of the car and they cost rather a lot more.