if your going to buy a car to modifiy dont just look at the car, look at the aftermarket parts/ support otherwise it could cost you later on many $$$$$.
180/silvia = cheap to modifiy.
civic/integra = cheap to modify.
supra/skyline = can get very expensive.
preludes are not cheap to modifiy (i know i have a prelude), it's more expensive to modifiy than a skyline/silvia and you get dont get the 'bang for buck'
i think the silvia/180 is the best way to go, for 5g (on top of the purchase price) you would have a 200kw + engine, basic suspension and brake upgrades and it would beat most of the cars suggested in this thread.
civic/integra can be 'hit and miss' cars, sometimes they can be made to go fast but it can cost $$$ to do it properly, other times it can cost next to nothing to make them go fast.
so make your mind up already.