Buy a shit car and pimp it up? Trust me you'll get bored after a while.
I love my Wira, dont get me wrong - but 85 BHP and endless endless understeer gets boring after a while.
Mod wise - depends what you want - my speakers & HU were an "investment" as I can (and did) move them from car to car, and my wheels still have a resale value - but engine mods etc (like my utterly useless K&N filter) are pointless as you cant really do anything with them and the gain is infentissimle.
While I know you wont listen (i didnt!), spend a little money on your car by all means but dont go crazy - save the big £ for another car.