I've acquired a Bushmaster Mod. XM15-E2S lower with a Timney trigger, and I want to use it to build a rifle. (It is presently part of an assembled rifle, but I'll be returning the upper, the stock and any accessories to the owner when I've got substitutes).

From what I understand, I can simply buy a complete A3 flat-top upper for about $400 (Midway offers the Del-Ton AR-15 A3 Flat-Top Upper Assembly for $399). I assume that I could just replace the current upper with that and the Upper/Lower assembly would be all ready for a stock and whatever sights I settle on. Am I right? Am I thinking the right way?

I'll also need a stock. I'm thinking of a MagPul CTR or ACS stock.

I assume that I'll need a receiver extension buffer tube assembly as well, and I gather that they come in two varieties: mil-spec and commercial. Am I right, and if so, does anyone know which fits the XM15-E2S?

I'd greatly appreciate advice: I realize that these are probably pretty elementary questions, but I'm brand-new to the world of AR-15s, and I'd feel much more secure about leaping into something if I could get the ideas of people who really know what they're doing.

Thanks.

UPDATE: DOH! Just answered one of my questions: the receiver extension tube is commercial (flat top threads).

Yes, you can just buy a complete upper, put them together, and you're good to go. I'm not 100% certain, but I'd say you need a commercial buffer tube.