Usual stuff really, nothing different from any other turbo car.

I presume we talking about a UK spec CA18 powered 170bhp job?

Big ends are often dead on cheap ones as they often 15y/o cars that have let a very hard life.

Rear subframe bushes often are shot, making a very floaty (or blatantly moving on hard braking etc) rear end.

Apart from that, tis the usual shit.

Ebay is your friend for S13 bits, front mounted intercooler is a good call as standard one is useless, same for exhaust.

T28 from an S14 or summat, 15psi, uprated fuel pump, stg2 chip, and the above mods, will give you about 270bhp

Bit more boost, 300ZX AFM, GTR/GTIR injectors, and stg3 chip can give you 300bhp or more, esp more if you got one of the bigger (but laggier) T28s like a GTIR one.

If you really want fun (unless your fun is gay drag racing, and if so, no point getting an S13) id not worry about the engine mods, and spend money elsewhere on it...