I reckon you will be able to pick up a Turbo for £4k, but that might be T or V registration kind of money - and I wouldn't expect full VW service history for that price??.
Also depends on where you live as (for example) cars down south seem to be a little more expensive. Loads of them about though, so do your homework and take a few for a test drive.
My cars got 87k on the clock and drives like new. It's had had VW servicing every 10k and had the cambelt and water pump changed (so as you're looking at cars with over 60k on them make sure this has been done - or budget £350 - 500 for the work to be carried out)
I suppose if I was going to sell mine I would probably ask for around £5,500.00

I got quoted £660.00 by Zurich insurance, but shopped around and got it down to £330.00 with Adrian Flux - Mind you, i'm an old git now (30) with full NCB....Putting the wife on the policy reduced my premium aswell

(but they have never had the pleasure of being a passenger with her!!

)
Have a look round this site for other tips and stuff, and the insurance comparison thread for other examples of insurance quotes.
Cheers
Stu