I would avoid the more modern Trek 820. it's hardly a "mountain bike" and more of a "mountain bike-ish hybrid." very flimsy bike.
keep scouring the used market. do your research and and jump on anything good FAST because the bicycle market is hot right now.
wheel size has almost nothing to do with how it fits the rider. my wife is 5'1" and rides a 29er. you just need something with a small-ish frame. I'd think most bikes with a frame labeled 15 or 16" should suit you, regardless of wheel size.
Last edited by mack_turtle; 10-20-20 at 10:13 AM.