1. Check its had the water ingress
problem sorted. Check the service receipts to find out. If theres no receipt check if the carpets in the footewells are damp, if the works been done or its dry, get in the car shut the doors and windows leave the air con off and check that it doesnt smell of damp!
2. Mileage is your enemy. Put up with an older car with less miles. These are german pieces of engineering, they dont rust, but for every mile driven those pistons have gone through that block so many thousands of times.
3. Service history. Theres too much to break under that bonnet to risk buying one that hasnt been treated well. Check its had the cam belt done, this is £350 job that you want done preferably.
4. Owners. Make sure the guy (or gal) you buy it off is a decent kind of person who wont have beaten the car and has looked after it. If its a private sale, ask to use their bathroom, if the house is clean and tidy then theyve probably looked after their car. Make sure there isnt hundreds of previous owners.
5. Mods. A car that old, with that many miles thats been chipped tuned and ragged round the circuit is a pile of cack. youd be better off buying a donkey to get to work and back.
Other than that good luck with the hunt. Mines an 01 with 55k on the clock and shes a peach, just a bit of a niggle with running lean, but i think ive fixed that now.
cheers!