I'm in the process of building an A1 now and have researched pretty much non-stop for a few months, the kits are hit and miss, it really depends on the vendor.
has a lot of info on correct parts by model.
These kits are CH forge marked and have a C/Bell FSB so that makes them 1974-82, they are likely 76-79, obviously post Vietnam parts, despite what the vender wants you to think.
Sportsmans guide sells them as 'like new' and they are beat to crap, I've heard good things about Whatacountry.com though http://whatacountry.com/colt-m16a1-parts-kit.aspx
These kits will be used regardless of the vender though.
AR15sport.com has correct style barrels in 1/9 with FSB attached so that's the best route for a correct build. http://shop.ar15sport.ihoststores.com/productinfo_v3.aspx?productid=BARREL-20LWT19
This is a big deal for retro builds as 99% of all AR barrels use a .750 FSB whereas A1s used .625s and the .625s are nearly impossible to find in a 20" configuration.
The A1 rifles used different lowers, which are available but are on indeterminant backorder from NoDak Spud, so an A2 lower wouldn't be a bad choice at this point.
In addition to a different profile, they are also a different color than modern A2 rifles, assuming you decide against the whole kit the best option if you don't want to do some refinishing is to order a refinished upper from Apexgunparts.com, they are done in a modern hardcoat anodizing to match an A2 lower at about the same price as having one anodized. Here https://www.apexgunparts.com/product_info.php/products_id/3922
If you do refinish the upper and lower to match, Norelles moly resin in Grayish black flat is the best option there as it's nearly identical to the old gray anodizing. http://molyresin.com
You will also need either a new LPK or to neuter the included one as the parts are Full auto configuration. I'd recommend a new one to ensure legality, Stock sets in the kits may or may not be in good shape, if you want very good furniture it can be had here at probably the cheapest price.
If you do order the kit, you can always sell some of the unused parts to recoup funds for replacement parts, I might know a guy
I'll link my thread for you.
Be aware, retro BRD isn't cheap (unless you do battlefield pickup condition) and it's highly addictive, I'm already planning another and will be getting a kit in a month or two assuming they are still in stock
The lower is a stand in until I pick up another. You can see the color difference.