im guessing sg=singapore? welcome first and foremost good choice of a peformance machine. Im from Malaysia, ...join the west malaysian forum...heaps of s15 worshippers there.
Owned the car for 4 years and touchwood nothing has skipped a beat. In short:-
1. Body: Car has very very defined lines and tight tolerances. Do the usual corner stand test in full light and check the bodywork. Thankfully in our region, crash repairs are usually done by ham fisted ppl, so any paint overrun or misalignment of body panels should stand out like a sore thumb. Doors should shut tight and not tap against the rubber staying ajar. Boot should align against the kink at the bottom of the rear windscreen and not offset. Even by a little would suggest an accident in the past from the rear.
Rust wise, as per what lupix said, also remove the spare tire and check the tub, it should not have rust spots hinting at waterlogging due to bad weld seals. Remove the jack and push your hand as far in as you can, a little sand is ok but nothing wet.
2. Engine: Stock car should idle nicely around 750-800rpm without any ticking sound. It should rev smoothly WITH grunt to 6000revs and drop off smoothly. AVOID cars with blow off valves that dump into the air (makes queer noises when you 'boost'). SR's need to have their blow off valves plumbed back. Otherwise you'll have fluctuating idles and high fuel consumption. Mileage above 100 000kms in our climate, have the dealer/seller change the turbo gasket (mentioned by lupix) and the belts as well. Look out for excessive black smoke during hard accelerating or idling or a black spot on the rear bumper where the exhaust tip is. Fuelling problems are imminent (usually associated with a standalone computer like the Power FC and bad tuning). The stock ecu is very very good for daily driving. Other than that, it'll take a nuke to destroy the SR.
3. Electrical: Bulletproof except for the airbag circuitry. Make sure the airbag light goes off after you start the car. If it blinks or stays steadily on, its normally a pain to fix. Power windows,,some of us have had to replace the drivers side motor by 120000kms...mine is slowing down already. It should go up and down reasonably quick with no hesitation
4. UNDERBODY: Cant stress this one enough...once you've picked one you like, get it hoisted on a 4 post lift and inspect the undercarriage. At the front, minor stone chips are fine...make sure the underbody cover (plastic protective layer) is present and secure. If its missing you can see right through to the bay and it could suggest a front end mishap in the past, repaired. Further back, the oil sump should not be deformed or bent.. move back and inspect the main frame rails. They should run dead straight with no kinks or flattening on spots. Minor ones should be repaired but stay away from major ones or worse, some that dont even have or are missing half the rail (normal on the rps13 silvia...not the s15)
Enjoy