I am also 6'5 and near 200lbs so I can give you my personal opinion. I find that 26 inch wheels are a bit on the small side, but they are ok for off road riding if you are trying to stay cheap. A good option might be to get an old hybrid bike what will take the larger wheelsize(700c=29inch), and will have a little closer to road geometry, sometimes they even have suspension forks(heavy cuz they arent very good quality). If you run 32c wide tires you will still have reasonable speed on road surfaces, but you will also be able to handle some pretty gnarly off road stuff. The hybrids are nice for off-road because they usually have cantilever or V-brakes like mountain bikes do. I wouldn't worry too much about a mountain specific bike until you really get into the sport and are willing to drop close to $1000 on something you will love. Once you are out there for 6 months or so you will learn what you like to ride and what kind of setup makes the best sense for you.

if you go for a hybrid frame look for something in the 58cm range, that will allow you the crotch clearance for offroad while still being large enough to ride comfortably(maybe with some small adjustment to stem length and handlebars). You can also go for an old MTB bike but it wont be as good on the road. Either way I would definately find a used one, you'll get more bang for your buck at $350 if you get a used bike rather than a new one.

Just remember that you will have a better experience if you dont select some POS Huffy type bike for $350 from the store just because its shiny and new. A used bike at the same price can get you something very nice. Just bring one of your biker buddies to take a look at it and help you find the right one. Craigslist rules for this reason.

If you want to stay MTB specific Look for a used Specialized Stumpjumper on Craigslist, they usually have some good deals on them and they are plentiful. They come in full rigid, hartail or softail and range from about $100 for a rigid 90s model to about $400 for a hardtail in good condition, you wont touch the softail for less than about $800 though. They make a great first MTB.