Contemplating my first build and have been really interested in putting together a rifle with an A1 upper. However, I am not comfortable enough with my own skills to assemble/headspace an upper on my first attempt

Do any vendors sell complete kits with A1 uppers and preferably Lightweight barrels?

Not sure about complete kits, but I think CMMG has the upper you're looking for in the EE under AR parts.  

You might check some of the parts companies too.  Sorry I haven't taken the time to learn how to make links hot yet.

www.model1sales.com
www.m-aparts.com
www.jtdistributing.com

I'd recommend J&T if they have what you want.  I've dealt with them before, good service and good quality.
Scott

[url]www.cdnninvestments.com[/url]

They have brand new complete Colt A1 uppers, bolt/carrier assys, charging handles, and stock assemblies. If you go that route, and mount the upper on a post ban lower, you'll need to pull the flash suppressor and put a permanently attched muzzle brake on it, and file the two little wings on the bayonet lug down so they won't retain a bayo (easily done w/a flat needle file).  That will give you the closest you can get to a Viet Nam era M16, and still keep you legal[:D]