Yer mate they are a quick as fark car but the driveline is what you wanna look at first before you even bother modding the engine... most people just convert them to front wheel drive as the FWD boxes take more punishment than the AWD box does. also the CV's love to die in them so factor that in.
parts should be fairly cheap as they are mechanichally identical to 1988 to 1993 323's and they share a lot with the 1st Gen MX5
The BPT 1.8 litre loves getting work done to it and simple work such as FMIC, boost, larger turbo and cams will yeild great rewards. if you want a freakishly quick one be prepared to spend money tho..
A mate of mine had the silver one with the chromies and a FMIC that took up the whole front bar and it used to wipe out cammed V8's and would walk all over my V8 :'( really humiliating

it was a great car till it broke....
To sum it up, they are great bang for buck but expect it to break if you drive it hard constantly
Hope this helped