I'm by no means an expert in XC, but I'll give you my 2 cents on some of the stuff you ask. I'm sure there will be others that can help you more. BTW, I'm pretty much in the same boat as you. Keep an eye out on craigslist too.
OK
1. What makes a cross bike a cross bike? - fatter tires with knobs, some like the middle levers, other don't, different brakes on the bike, and different measurements usually on the frame tubes than a road bike.
2. How can I find out what is adequate components if I were to build a bike from a frame? and how cheap could I do it? - read, read and read $600 is a doable mark
3. Would a Bikes Direct cross bike be adequate for my current needs and be worth upgrading to better components in the future if I wanted to? - Not crazy about BD bikes but I do keep an eye on what they're doing. I do like Nashbar frame and carbon fork. Bout a $250 option right now. Does go down if you watch for it.
Good luck on what you are doing and I wish you the best.