Fun to think about all the great options. Long story short, I'd probably go with 7 PRC. I suspect it'll maintain popularity and make finding components easy enough in the future. 7 SAUM, 300 PRC or 300 norma would be cool too, should be plenty of brass available for each. I think the some of the best advise out there, as discussed on Shoot2hunt and talking with guys at UM, is think about what your goals are, what animals do you want to shoot (elk), how far do you want to shoot them (up to 700), what bullet or class of bullet do you want to shoot for the given caliber, determine how fast you need to push that bullet to allow it to perform at said distance given barrel length considerations, and how much recoil do you want to put up with? When you work through that process and running numbers through a ballistic calculator it answers a lot of questions and also makes you realize there are lots of great options! I'm intrigued by the 6mm, 6.5mm threads. 6PRC or 6UM could be sweet too. Reading some posts from Frank at Bartlein about generic info regarding burnout shot counts for various cartridges, maybe just order a couple barrels at the start lol.
The BC on 180 ELDM's is awesome, G7 around 0.4. Standard atmospherics, start that baby at 2700fps and its still hanging onto 1957fps at 700 yards. Should get the job done. More speed gets my titties hard though, if my chrono said 2700, not going to lie, I'd be a little sad.
The BC on a 215 Berger, G7 around .354 (recommend custom drag models), start that at 2700 fps and still doing 1870 fps at 700. Again I'd want to blast faster but just gives an idea.
The BC on a 115 DTAC, I think I've seen G7 around .293, start that at 3200 fps, still blazing at 2114 fps at 700.
I can't speak to shooting elk with fast 6's or 6.5's since I haven't done it, but I do like pictures and seeing what bullets do, so hope they keep adding to those threads.
Good luck and enjoy the build!
