No personal experience with the 6.5 Grendel, but were I to venture down that path, here's what I would consider doing:
1. Get just about any reputable stripped upper. Forging is pretty much commodity now, given the high quality of CNC machining. Get the BCG I prefer and the rail I prefer, but it would need to be free float design. I understand that the AR bolt extractor may be a bit weak for the 6.5, so either a dedicated bolt or upgraded extractor.
2. Get the best barrel I could afford, if long range accuracy is the objective.
I don't want to single out any one company as better than others, but you can do this for well under the cost of any decent AR-10 and get a really nice custom barrel.
I will give an example, but any of the top barrel makers would be comparable. I have been very happy with the drop in AR15 16" M4 Navy Recce barrel I got from custom barrel maker, Lilja. It was $480, plus shipping from the manufacturer.
They do not have vendors. You deal directly with Fred or Carson Lilja. That barrel shoots sub 1/2 MOA with my match handloads. The practical limit being me and my mediocre marksmanship, even at bench.
Lilja makes several drop in barrels in 6.5 Grendel. I'm sure the other top custom barrel-makers like Kreiger, Bartlein, Shilen, etc., do the same. Be forwarned: These barrels will make you want to upgrade other stuff, especially the trigger group, and may lead to handloading if you are not already there.
I cannot address magazine compatibility issues.