Fit is most important, Go to a local Specialized,Trek etc dealer and get matched to what size bike frame is best for you then look for a used one
I learned the hard why on buying brand new in this situation by buying a brand new Trek 7000 model for the average going price of one thousand bucks in 2000 ,,,,,,I was at a flea market a few months after buying the Trek and came across a used Marin (USA made and top rated at the time ) mountain bike which retailed for 700-800 brand new ,,,It was a 1999 model in near perfect condition and i got it for 55 dollars ,,, Needless to say i enjoyed it more than the Trek