Have a look on Autotrader, I am sure you can get the same for cheaper, however, the mileage for that age of car is fairly low.
Regarding the cambelt - no, it doesn't necessarily mean that it has been thrashed. If it's going to go, it's going to go... and when it does, it's messy and expensive.
If you are to think seriously about this car, check what has been changed when the tensioner went. If they used all reasonable parts, the engine will be like new and you won't have to worry about the cambelt and stuff for the next 80k, however, for the sake of £250, I have mine changed every 50k... just cos I prefer preventative maintainance.
Also check that the water pump was changed at the same time!
Apart from that, a genuine 75K 1.8TS will be fine... Mine's just hit 100,000 miles this week and is running lovely. Just replaced the Diverter Valve too, £13, and the ride is much smoother.
Good luck, but remember, if this one isn't right, walk away, there are 100's on 1.8T Sports about...