Unless you are also going to start re-loading your own ammo, or have connections at Silver State Armory (for 6.8 SPC - Silver State Armory manufactures and sells 6.8 SPC Ammunition and custom brass cases, 5.56 ammo, 7.62x51 ammo
), 'no' they aren't the only ones who supply it, but one of the main ones - I would suggest staying with 5.56 for cost (cheaper), availability (more widely available - and in more weights and type of bullet), accessories, etc.
You can always add a complete upper in one of the other calibers (6.8SPC, 6.5 Grendel, .204 Ruger, 7.62x39, 9mm, .450, .458 SOCOM, etc) later.
As for barrel length and gun configuration, that is something you need to decide, as far as what type of shooting you'll be doing most, if you'll be doing more longer range shooting - then you'd probably be better off with a longer barrel (18-24"). If you are going to be setting the gun up for close quarters, mainly shorter range target/plinking/etc, then a 16" barrel would probably be a better choice.
Mainly it comes down to what you want it to be able to do, what you want it to look like while you are doing it, and how much $$$'s you want to put into it, whether you want to buy it pre-built whole, pre-built in separate assemblies, or build it yourself from parts.