I'll pipe in even though I am much less experienced mountain biker (5 years) than a roadie (27 years)...
As for the bike, go used. Like cars and boats, the early depreciation is heavy and there are lots of bikes out there that were bought new, ridden a few miles, packed away in a garage and finally put up for sale. Don't pay more than 50% of the new price even if the bike is mint. After buying your bike, take it and your wallet to a local bike shop to get the other things you will need (shoes, helmet, water bottles, shorts) and have the local bike shop look over the bike and adjust it to your specs. They may charge you a little for this, but that is okay. The LBS will work with you if you buy your other stuff from them.
As for brands, there are bunches out there. But, beware that Trek makes all models of bikes and just because Lance Armstrong rode one that doesn't they're all top of the line. Actually, I've seen some really cheap looking stuff with Trek labels. Giant provides good value for the dollar. Specialized is probably a little more than you need. Stay away from any bike that was purchased in a department store or big box like Costco.
Stay away from any "mountain bike" that doesn't have front suspension. These are actually hybids and don't do anything well. Your choice will be between a hardtail (no rear suspension) and a full-suspension. The latter are more expensive, so that is a consideration. A hardtail will provide a perfectly good ride but if you get the chance to snag a full-suspension for a good price that is even better.
As for the equipment, Shimano (figuratively) owns the mountain bike equipment world. Deore will be fine for you. If you get the chance to go one step up and get Deore LX, so much the better. As long as the bike has cleated pedals, it will probably have equipment sufficent for you. Any bike with metal or plastic pedals should be avoided.
As for disk or caliper brakes, both do the job of stopping you. I have v-brakes, but am considering disks. As for K2 equipment, I have a K2 bike, 1999 1000, and it has Deore equipment so I don't know what K2 cranks are.
Most importantly, get a decent bike, make sure it is adjusted for your size, wear a helmet, don't get crazy and break your collarbone, have fun and burn some calories!