Welcome to BF!
Pass on this one.
While you are getting yourself back into bicycle buying mode my advice is to think about what you really want to do with the bike. You and I are the same age so no ageism here but I doubt you really want a mountain bike but rather a gravel or hybrid bike. If you are looking for something mid range quality wise used is going to be about $500 unless you get very lucky or know what you are doing.