Firstly, there are no stupid questions when you're about to plunk down $250 or more on something you know very little about
I have neither the Slash nor the Stampede (both are great RC's).
1)My suggestion would be to first find out from your son what kind of vehicle he wants (stadium, monster, truggy, buggy, etc.). Is he interested in eventually racing?, or just bashing around? Use YouTube to look at videos to see what kind of terrain and driving you all like. ALSO, decide if the type of terrain they're driving on is compatible with that where you live.
2)Many folks start off with brushed motors simply from an initial cost standpoint. Once they decide to stick with the hobby they upgrade to brushless. Brushed Traxxas RC's are also waterproof (so are VXL brushless, but at a price premium). If you do not go into water it's neither here nor there.
3) Most 4wd's by their nature do a little better in rough terrain and grass. This is sometimes at the cost of shorter run times and a little more maintenance. 2wd's can really scream on dirt and asphalt (and off road too).
Another thing to consider is batteries, Traxxas RC's usually ship with batteries and an overnight/slow charger.
You also have a choice of NiMh or LiPo batteries.
You'll also eventually want a "fast" charger if ya'll get bit really hard by the RC bug.
You seem to be off to a good start with your research so far. As far as online vendors, I do not have a clue. I do know that E-bay has been a great resource for spare parts at reasonable cost. My nearest hobby shop is an hour away.
Research some more and fire a few questions back