A colt upper receiver kit would be your most logical choice, for mfr continuity, finish matching, receiver fit, and a chrome-lined barrel. You can easily find one of these in the shotgun news; R-Guns, CDNN, Ky Imports, JT Distributing all sell these at around the same price ~$400 +-$50. Apparently, as we debated on this forum the mid lengths are en vogue these days and if I were to start totally new then going with one of these would be prudent.
(I have a carbine which I am totally content with). The mids are apparently available from Armalite or Rock River.
As for slings, I have used an Eagle Industries tac-sling, a 3-point type like an H&K MP-5 sling. I've taken it from the deserts of 29 Palms, to the snows of Bridgeport, CA and now in Karbala, Iraq from where I am typing this. (I am stationed in CA). Its a great sling for around $30. Another good one I've seen lately is from Spec-ops gear. it has some elastic bungee in it so you can muzzle tap someone (if you so desire). Best of luck, that's my scoop!