I've been a 330 lb bike rider before. A good bike is a good bike. Don't drop $1000.00, you don't have to. Do some reasearch online, look for reviews of bikes and look form something that is recommended as a quality bike, that is in your price range. Don't worry about the weight, you are not going to find any info regarding that. Worry about getting a good bike. You don't have to break your bank, you might find that you hate riding and want to do something different. but I'm guessing you will spend $300-$600 for a decent, quality bike. I picked up my Specialized hybrid for about $150-off on sale, and I got my moutain bike for $700, down from $1100 at a "we're not carrying this line of Bikes anymore" sale. Get a mountain bike if you want, but unless you are going to actually do some serious trail riding, get a decent hybrid. Fine for mild trails and still great on the road. If I hadn't bought the MB first, without really thinking about my needs, I would have just bought the hybrid.

You can put mountain bike tires on it if you want to, but you probably don't need to. Hybrids have a better seating position too, especially if you don't have a strong back yet, and hybrids are pretty rugged. You don’t need a bike that can be launched off a cliff. Don't worry about supension, if it has it, that's fine but don't worry about it. My hybrid has a shock absorbing seat post, and in all honesty, at 280 lb currently, it still has a hard time working with me. I might switch it out in favor of a more standard post until I lose another 20-30 lbs..

If your bike doesn’t come with one, get either an anatomically correct saddle (different for men and women) or get a nice spongy gel seat. I have the gel on my hybrid and need to get an anatomical one for my mountain bike. I also have some gel-padded cycling gloves that I really like.

You're going to have to watch the tires. Buy a portable frame-mounted pump and get tires with schraeder valves. These are the usual standard valves and are more universal then the other kind (whose name escapes me at the moment). You can pump up a schraeder valve tire at pretty much any gas station that offers air. The tires are gonna squish down when you ride and make a bit of extra noise, but as long as you check them to make sure they are good and pumped up before you start, and then every now and then if you stop for drink or something, you will be fine. Just don’t let them get too low. If you do you will bend your rim, no matter how strong it is. Also, get a little pack and stock it with a multi-tool, tire irons, and a patch kit.

Get a couple of frame or ahndlebar moutned water bottle racks, or get a camel pack. If you are new to riding and it’s the summer, then you need to stay hydrated and you won’t necessarily be able to get to water when you need it if you are out on the road.

Other than that, there really isn’t much else to say. You are right that bikes aren’t aimed at people as big as we are, and somewhere out there might be a bike that you can buy this is, but it will cost you an arm and a leg. IMO, you are better off getting a decent bike and just keeping up with the things that you will need to watch. The worst that will probably happen, is that you may go through more tires and maybe even a rim if you are not careful, than your average rider, but that will change as you lose weight.