The older frames that I like are the Giant VT’s the Spcialized Enduro’s from 2003 and 2005 and also the 2005 onwards enduro 150. The Kona Dawg’s are also quite good frames when fully serviced.
If you buy a whole bike it gets tricky, realistically you’ll need to check and more likely than not service, everything from the brakes to the forks also the frames bearings should be checked as well as the rear shock.
If you lived locally I would give you a hand to source and check what ever you buy. Then build the bike up, but since you live in England you’re a bit too far away.
My advice is buy all the parts you want and build yourself a bike, that way if you have to make a claim on feebay or check out parts when you go to view them. You don’t have to check everything on the spot and won’t potentially make a mistake that could be costly.
A whole bike simply means a lot more servicing and a lot more to check!
There are two professional traders on ebay that brake up bikes all the time and sell them. You might be able to buy a good condition used drive train from them.