As a few of you know, I'm a big fan of the 7mm-08 as an all around big game rifle. Also I'm a big fan of the AR15 as a varmint rifle.
That being said I have two hunts coming up next year for Wyoming pronghorn and Alaska dall sheep respectively.

Using that as an excuse to give my wife I'm about to start building an AR in 7mm and need some help finding a barrel and action to get started with.
My total budget for this build is around 2k but as far as larger caliber AR I'm a newbe so any and all advice is welcome.

Will need to be a 308 ar platform. An ar-15 can not fit a 308 or 7mm-08

I would use the mega combo receivers

Mega are nice. But spending money on the receiver...

I got a custom spec'ed McGowan barrel for my 7mm08. Other options are Kreiger and Black Hole Weaponry.

You might get away with using an AR10 for pronghorns, but take a hint from experienced sheep hunters and consider something lightweight for hunting at high altitudes in the mountains.

I got an 7mm08 upper and lower for around $1100 from Palmetto State Armory, could not be any happier with how well it shoots.

This will be my fifth sheep hunt and I also have 4 mountain goat under my belt.
I'll be taking my tikka t3 as my primary rifle, just don't tell my wife

What distances are you looking at? 6.8 SPCII is a lot of gun in a light package. Any
20" 7mm-08 would be much heavier for maybe 400y gun vs 300y 6.8 (medium game range).

The main purpose of the gun will be pronghorn and bean field whitetail out to 400 but also I like to take two rifles when heading out of state so it will be a backup in any situation you can think of except moose and brown bear. Hunting mostly texas Idaho and Alaska.

If you're giving the benefit of the doubt to the 6.8 at 300, then a 7mm-08 is easily 500-550, not 400. Federal 7mm-08 with the 140gr Nosler BT has more energy at 500 than the Federal 6.8 MSR 115 gr does at 200. Granted that's from a bit longer barrel than 20" but it won't make that much difference. The 7-08 packs a serious punch. I have both and the difference is very noticeable.