If your lower is a preban, you can put on a tele-stock. If not you're restricted to a fixed stock by the '94 crime bill. Olympic makes a fixed tele-stock if you're just wanting the look of a pre-ban.
The tele-stock is handy for getting in and out of vehicles and aircraft and the four position adjustable ones make getting the right length of pull handy for women and small statured shooters. But, they aren't the best shooting platform.
As for barrel length, Rob is right, unless you want to jump through the hoops and have a registered SBR your overall barrel length has to be 16". Many companies sell 10 -14.5" barrels with a flash hider or compensator permanently attached that brings overall length to 16".
The 10 and 11.5 inch barrels with the permanently attached flash hiders (legal on pre-bans only) are loud and uncomfortable to shoot. You also lose a lot of muzzle velocity with them. If you have a preban lower the best compromise would be a 14.5 M4 barrel with a permanently attached vortex flash hider. Bushmaster has these in 1:9 twist for $199.00. I think YTM has an extended A2 type flash hider for this application as well.
Colt is selling 16" M4 barrels, these barrels are 16" without the flash hider, so overall length would be an inch and a half or so longer. You also can't mount a bayonet on a 16" barrel, but you can with the 14.5. This is important to some people.
I'd stay away from the 10 and 11.5 inch barrels, unless you are just wanting to have a rifle that looks like the old XM177 series. If you are interested in building and XM177 clone, make sure you get a post ban front sight base without the bayonet lug as the XM177s didn't have a bayonet lug.
HTH
Jeff