1. The most important thing about buying a new
bike is to make sure it fits. The only way you'll know if the bike is right
for you is to size up the bike and make sure that the bike's geometry matches
your body's geometry. Ask questions and do some research.
2. If possible, try to find a shop that will
let you demo the bike on real dirt. Five minutes in a parking lot won't cut
it. You wouldn't buy a car without a real world test drive, and a bike should
be no different.
3. Don't belive the hype. Just because
your favorite rider or best friend rides a certain bike, that doesn't mean
that's the best one for you. Have an open mind and be realistic about your
needs and ability.
Not sure what you're looking for here. With enough tinkering and a welding kit, all things are possible.
I thought you already had an adult-sized frame?
There are two things that determine what wheel sizes will work in a frame: the brake mount and how much space there is. So step one is to figure out if you can even fit a larger wheel in there. The tire is where you may run into trouble.
Supposing that fits, if it's a disc brake bike, you're golden. A fair number of people converted their 26" hardtails to 650B that way.
If it uses rim brakes, you have a problem. The range of adjustment on rim brake arms is not that great, so you may not be able to land the pads in the right place.
What kind of bike is it? What kind of brakes? What are you trying to make?
"Don't buy upgrades; ride up grades." -Eddy Merckx