Well, first of all, welcome.
Second... are you capable of working on your own car? If not, plan on learning.
Third, while looking for a MKIII, check for all the usual things. Smoke at startup, during acceleration and deceleration. Check the coolant for any signs of contamination (look in the radiator and the overflow tank). If you're capable, do a compression test and read the spark plugs while they're out. If you aren't equipped to do this get a mechanic you trust to do it. If the seller says they already tested it, do it again, it doesn't hurt and you don't know if whoever did it previously knew what they were looking for.
A lot of the things you need to look for are pretty standard. The MKIII prone to having headgasket issues, due mainly to the low factory torque specification. So focus on that.
Welcome to the forum, if you have any other questions try a search and if you don't find what you need just ask for directions.