I'd ask on some MTB specific forums such as single-track. I know Trek do some very large sized bikes, but whether this extends to their MTB range I don't know. Unless you come across a deal I'd go hard tail for that budget, but you will get a nice hard tail for that price, plus of course a bit more multi purpose than a full suspension bike.
If you ever decided you wanted a more general bike then Surly long haul/disc truckers are pretty bombproof and they make them extremely large.
General cycling advice ( NOT technical ! )