BSA basically lied about updating the regs on these, so the regs in the latest ones are as crap as the regs in the early ones, which means for the vast majority of R10s, they will fail, if not soon, then later on, it's a numbers game with these rifles.
In saying that, once the regs are sorted - not by BSA of course, then they are no more trouble than any other PCP, and generally give good service.
So buy the latest and nicest based on looks and condition, but most of all, get one a bit cheaper if it's got a standard reg, or pay a touch more for a gun which has been sorted by a respected tuner, or at least had a decent reg fitted to it.
Realistically, I'd be looking at around £550 max for a standard one, and around £600ish for a fettled one, they do come up at these prices, and the ones you see at £800 always "go back in the cupboard"