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.
It's also worth giving your wheel a spin and seeing if it's still running true.
One of my spokes has been bent for close to a year. It happens to bikes that get ridden off-road.
Pluck the spoke like a guitar string and compare the pitch to the neighboring spokes. As long as the pitch is similar you should still have a relatively true wheel. If it is a lower pitch, grab a torque wrench and tighten it a quarter or half turn until the pitch is similar to the neighboring spokes pulling from the same side of the hub (ie, compare nondrive side spokes to one another, drive side spokes will always be tighter/higher pitched).
Not much to worry about. Since I built my wheels I would deflate the tire and demount that area on one side. Clip the spoke 4" down and push it and the nipple out. Weave in a new on and reverse thread a lubed nipple onto another spoke and poke it through and start it unto the new lubed spoke end. Bring it up to tension with the TM-1 or by pinging. Cost a buck and take less time then this.