Replying to Mountain bike recommendations

Looking to get back into mountain biking and need a new bike. I used to do it a ton quite a few years back, but it's been a while. Planning on doing downhill/freeride/xc on this, but don't care too much about weight, I can definitely work with what I get. My budget is around 800 or so for the bike, maybe more if I really have to though

I was planning on going used, and I've seen a few kona stinkys on pink bike/craigslist in the 7-8 range, from around 2010. Would this be a good option? Or what other ones should I look for?

Also sizing... I'm 5'10. Medium or large. I talked to a guy at a shop briefly and he said I could probably go either, so which would be better (or bring them a 6er and get actually sized)