The S3 is either an 8L or 8P model. The 8L was produced from 99-2003 in SA with very few registered only in 2004. All of them featured the same basic 1.8 VW/Audi group engine block used in various models such as the VW Mk4 Gti, Passat 1.8T, Sharon 1.8T and Beetle 1.8T as well as the A4 1.8T, and A3 1.8T, however the internals differed substancially to deal with the larger boost of the bigger turbo charger. They had twin intercoolers, but none of them were twin turbo charged from the factory (a myth linked to the naming of the Audi TT which had nothing to do with twin turbo)! Initially the engine produced 154kw. The motor was upgraded in 2001 to include variable valve timing along with a face lift, but the power remained unchanged. In late 2002 power was increased to 165kw matching the power of the identical motor fitted to the Mk1 Audi TT.
The most important things to look at on these vehicles are:
The Clutch (15-20k dealer replacement)
Brakes (5-8k dealer replacement)
Turbo (big money!)
Haldex coupling (or torque transfer/ 4x4 diff - 30k+ dealer replacement)
Gearbox (big money)
Stay clear of chipped and modified ones. They are sure to have been abused. While the vehicles are pretty bullet proof, no one wants to buy a vehicle that has been raped for the previous owner's thrills.
I would also avoid the pre face lift ones.
Because they are Audis, they have 12 yr anti corrosion warranties, provided the terms of the warranty have been complied with, but they rarely rust or give body work issues.
Get an Audi 80 point check done on one with a full Audi service history before buying is the best tip I can give you. If the owner couldnt afford the Audi service costs, you don't want to go near it.
In an S3, you get what you pay for. A cheap one is likely to have had previous damage, existing mechanical issues or a load of non-original parts fitted to it.