i'd be very surprised to find one for £5k
like rhyso says expect to pay around the £7k mark depending on options age and mileage.
best bet is the start looking now so that you can get an idea of what you'll be getting for your money.
mileage isn't really an issue with the derv's as long as they have the full service history, i'd expect them to have about 80-90 on the clock as the average driver of these would have been doing about 15K a yeah. The 150 was available from 2001 on.
also if you want to spend a little more you could try and find an anniversary (limited to 900 TDI's and 900 1.8T 180) but i'd expect to pay closer to 10K
if you've only got £5k to spend its probably worth considering the 115bhp version (can be mapped to 155 odd) or you could get a decent 1.8T 150 for that (but then thats petrol

)