Spade, it's not hard to do. But if you want to save some cash, it does take a little time and patience. Basicly start hitting the for sale boards and equipment exchange boards (the one here is great) for a good deal on either a preban lower or a complete rifle. If you go with the complete rifle, just yank the old upper off and post it for sale on those same boards. It may take a day, it may take two weeks but it will sell.
To add to a couple of things. I would definetely go with Bushmaster or Olympic (Bushmaster is the best bet if you can find a good deal on one).
If you go Colt on the lower and you don't know what you're looking for you could end up with one with large holes, or without the lower rec. improvements.
Also, I'd second the suggestion of the 14.5" upper with perm. attatched flash hider.
On the upper receiver, I'd go Bushmaster for sure. If I'm not mistaked they're the only company that are offering chrome lined chambers AND barrels. A must on an AR if you ask me. I have heard that Rock River Arms is starting to do this for a decent price now too, and their uppers are pretty nice too.
Good luck, and hope this helps.