First of all CONGRATS! I too found one a 6600 a few years ago unfired and pristine. I bought a Colt M4 upper for it before they became astronomical, but keep it in it's original condition- just too damn sharp as it is. That being said I assume it is Colt Gray, so the upper will be black unless you build your own. Anybody's upper should fit, and other than Colt, Bushmaster and Rock River also make sharp stuff. I think you probably have a large hole front pivot pin, which would need an adapter for a small hole upper. See www.biggerhammer.net/ar15/uppers
for more on that. For the stock I have only had Colt 4-positions. I like the "new style" ribbed six(?) postion. They are awfully proud of them but the Colt ones I've had are tight like a drum and fit nice. Try www.sawlesales.com/
for that. FWIW I am certainly open to other brands if they pass muster, but other than the inherent idiosyncrasies in Colt products they have been second to none for me. You must consider what makes YOU happy! If what makes you happy is twice as much and you can afford it- do it! If you need to balance the budget but still want quality go with the Bushy or RRA. Either way works. Congrats again and Enjoy your purchase and your liberty!
Edited to add- a loose telestock in the closed position can be cured by a simple o-ring slipped over the tube. I've seen it done and it works great!!