I had a similar conundrum recently. I thought about it and I end up pulling out the FCG, safety, stock, and grip on the lowers.
So I decided to build. No reason to buy a gun and toss out 1/2 the gun the day I buy it. I do buy uppers though bec I can get what I need at reasonable prices. I have DD, Bravo and Sig uppers. I like the newer BCM's with keymod and m-lok rails. Ran an 11.5" BCM in a class in Dec and it was dream compared to the DD MK18 upper I've been running the past 2 years. The Sig 516 is a tank. That one I'll probably sell.

That said resale value of a frankengun is nill. So if you sell guns often to offest the cost of new guns then buy over build. Either way The CMMG lower can be stripped, good parts swapped in and an upper thrown on. It just wont be a KAC....