I think with either it's a matter of getting the right bike. I got a second hand bike recently, had been pretty well looked after and the bits of spec I would've changed had mostly been changed anyway, forks had been not terrible but not great, it had Pikes on, it had a spare rear shock etc. etc.
Anyway, spent £1k for a decent 140mm 29er trail bike. Since then the dropper post has gone and I've put a shorter stem on, total cost £110. I've changed tyres over too, but that's the weather not the bike's fault!
Edit - I did get lucky in that exactly the bike I was after was available at exactly the right time in my size. Whatever you do, don't go for the wrong size - I have in the past and the bikes haven't been "keepers".