The major sources of weight in the AR upper are barrel profile, then the FSB. Go pencil on the barrel and find a lo pro gas block that you can work with attaching the handguards or float.
After that there are some tweaks using a slick side upper but for the most part they don't shed much weight for the additional cost of the smaller production runs. An A3 railed upper will be slightly lighter than the older carry handle types.
If a free float seems to be in the budget then avoid the ones with milled rails on them as they add weight and in some cases extra feet of unused rail. An ALG tube with quick mount holes is one of the better lower cost and light floats on the market. Be very careful choosing muzzle devices as anything more than a standard A2 flash hider goes up in price very quickly. Many of the designs are actually heavier, too. Keep weight off the end of the barrel and it will feel lighter than one with a lot of stuff out there that actually weighs less. For the average shooter, BUIS, slings, vertical handgrips, lights, etc add weight and cost, but they can only offer a very small incremental increase in accuracy for the expense. Which is something that practice can overcome.
As for a 14.5" welded and pinned, it makes any future changes difficult and the weight savings is minimal for the extra cost and 1 1/2" shorter overall length. If someone wants short, then an AR pistol or SBR with 10.5" barrel does that job better.
The internet is your friend, finding parts is as hard as finding a search engine like this one:
http://arpartsfinder.com/
