Originally Posted By overclock:
I no i want a colt lower.
I hope this is a typo. Colt is famous for being the only manufacturer to use non-spec part sizes for its lowers. The older pre-ban Colts uses a non-standard pivot pin size, which makes it difficult to use standard uppers. Current post-ban Colt lowers now use the smaller, standard-size pivot pin (well, actually, a screw, but at least it's the right size), but they use larger, non-standard pins for the fire-control parts.
I strongly recommend a Bushmaster lower, because they are the best you can buy. They are machined from forgings (not cast, like some of the junkier lowers), and more importantly, have top-notch machinging and Mil-Spec hard-coat anodized finish. Having said that, the lower is the least critical portion of the rifle mechanically (assuming the machinging was done right and the pins line up properly).
Anyway, any upper (except for old, large-pin Colt uppers) will fit on your AR15 lower. There are tons of upper choices. Any configuration you decide on will be a compromise, so you have to figure out what you want to accomplish with the rifle. The configuration should be determined by the rifle's role, with the compromises favoring that role.
The primary advantage of the AR15 is that the upper, containing both the barrel and sighting systems, is a self-contained unit that's easily and quickly replacable. That allows you to modify the rifle by simply swapping uppers. You can have a short, lightweight, iron-sighted carbine upper and a long, heavy, free-floated, scoped upper for the same gun.
The best advice I can give you is to go to the Articles link (just below the ads at the top of this page) and read as many as you can. Then read some of the other forums. The more you read, the better decisions you will make when putting together your rifle.