First, who built your lower assembly? It is usually a good idea to get the same brand for the upper. However, most all brands will fit each other but some don't. Different take-down pin diameters for instance. The tighter the fit between upper and lower, the better.
It is really easy to put an AR together. Installing the front sight base being the hardest to do. You can get un-drilled front sight bases and drill and tap for two setscrews up from the bottom to hold in place. I have done this on my competition AR. Or if you get a front sight base that is already drilled for taper pins but the barrel is not, you can tap the holes for set screws (4) and secure the sight base that way too. Sighting in the front sight for zero windage would be the only issue.
For tools, books and parts, Brownell's is your friend.
http://www.brownells.com/
BTW, you can never have enough AR. Just sayin'