Matt - I'm sure you'll find plenty of threads discussing buying pointers for S2's (and 944's in general)
Have you got a link to the S2 you're looking at - sounds nice (is the mileage really that low? - these cars are built to do high mileages & most of them are much higher than 57k)
If it is a very nice example, then maybe you won't have to worry to much, but just in case -you should find plenty of threads on the following:
Camshafts / sprockets / chain / tensioner - need checking for wear / damage & can need replacing (specific to the S and S2)
Rust - inside sills, bottom of arches
Clutch is an expensive replacement, so recent clutch is good
Caliper plate lift
If it hasn't been used much recently, the electrics could be a bit dodgy - check all switches, windows, sunroof, mirrors etc..
Vibration at idle usually means engine mounts are shot
Check for damp carpets & water in the boot
Belts should cost around £3-400, but you probably want to get the waterpump done at the same time, & may need oil seals & rollers replacing.
However good it looks, make sure you have some cash for the inevitable jobs a 20 year old car will need. they are great cars though. Drive a few & you'll soon see the difference between a good one and a tired one [
] (and then drive a turbo & see if you really want an S2 [
] )
Ed