>>629217+1 to hardtail.
If you've got offroad experience, especially MX, then you're going to do fine on a hardtail. Gear changing is probably going to be tricky to get down as you've got to keep your legs moving while you shift or you'll bind up your drivetrain.
Bicycles are so mechanically simple, I'd just do some research and buy a used bike, and upgrade what you need. Usually just fresh pads, new chain, maybe swap out some chain-rings when you get a feel for the ratios you want. New grips/cables/saddle go a long way toward making a bike look newer or well taken care of.
People have been shredding shit up on trails since bikes were heavy steel cunts, don't let people tell you all the shit you'll need.
Just get a well functioning bike, learn how to adjust cables for them silky shifts, and go out there and just start riding, after you actually get out there and get the experience, THEN figure out what high-end parts you want to suit your style of riding.
Craigslist is a great place to start. Just keep to used bikes of better brands: Trek, Gary Fisher, Haro things like that. Just avoid mongoose-tier shit since those cheap bikes used small threaded forks you cant upgrade, the frames are shit, and the crank is usually 3 piece garbage.
Pic related, built completely from used parts, ended up having maybe $250 into it.
