There are a lot of horror stories about the mk3 gearboxes, and it's understandable that it could put someone off buying one. However, many many people have never had any trouble with their gearboxes, regardless of age and mileage.
Unfortunately there is no hard and fast rule as to which ones are more prone to failure - it is just pot luck. But don't let it put you off buying one - just bear in mind that there is the possibility and perhaps budget it for it, worse case scenario.