What Jason says is correct over all aspects of buying a bike, cheaper is not better on trails, and buy locally.
I have a Specialized Rockhopper which 3 years ago was $450. When you go to a bike shop and get a "mountain" bike, which is not a hybrid from chain stores, you are going to find the prices to be anywhere from $400-$1500 or more. Do you need to go spend 2K on a bike that you are going to beat through the woods? NO not really.
I've done downhill biking and the things I would focus on are the brakes and tires. When you get a bike make sure you get one with, at least, disk brakes. Regular rubber brakes work ok but they tend to lock up more, which is bad going down a trail, and they are a LOT more work on your hands and forearms from having to squeeze them so tight. Make sure the tires have a good-aggressive tread pattern. This also helps with control down the trails, but when the trail turns to mud it doesn't matter what kind of tires or brakes you have just hold on for dear life.
Suspension is not a big factor, it's more personal comfort. My Rockhopper has front suspension only, my buddy has a full (front and back) suspension and I beat his butt all over the trails.
I hope this helps, if you have more questions please ask.
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