Hello and welcome!
I think you'll struggle to find a TD5 for £5k. That would get you a pretty solid, late 300tdi, though.
There are plenty of threads providing advice on buying and what to look out for, so my advice would be to start by reading through them and learning as much as possible. There's a nice 200tdi conversion in the For Sale section, but it doesn't sound like the sort of thing you're looking for...
Your best bet would be to shop around on ebay etc and find a knowledgable and experienced person to take with you when you go to look at a vehicle. But, like I said, there are plenty of threads that already contain thus advice, so take a look at the search function and stickies before you do anything else.
Good luck with the search!