thanks a lot of the help, can i put a 6.8 upper on a 5.56 lower if i wanted to switch out later or do i need to get two separate rifles?
Yes, the lowers for 6.8SPC uppers and 5.56 uppers are the same.
6.8SPC and 6.5 Grendel, however, are kind of boutique calibers. You aren't likely to find either at your local gun store, so you are more than likely to be mailordering in bulk for your ammo purchases.
There are only a couple of guys in my area that have 6.8 uppers, and they don't shoot their rifles. Nobody here runs 6.5.
At least one of these calibers is predicted to die out. Keep that in mind. I'd love for one of these calibers to become widely accepted, but until it does through a large military or police purchase (think of the .40S&W case), I'd hold off.
Now, stuff like a .308 upper is what requires a different lower, due to the length of the cartridge.
Get either a 16" or 20" AR-15 in 5.56, flattop upper, with a 1/7 twist to shoot the heaviest/longest bullets (you will easily shoot the lighter bullets). 1/9 twist makes no sense...1/12 twist is too slow. You will likely be using 55grn FMJ for most of your plinking, but it's nice to be able to use the heavier ammo (like 75 and 77 grn rounds).
I like LMT, Charles Daly, CMMG, and S&W for ARs. I do NOT recommend buying the cheapest AR you can buy. Buy once, cry once. Most of the cheaper brands are assembled with little attention to QC, or made of cheaper materials. Some of the more well known brands have prices that you could have bought better for nearly the same cost.
www.m4carbine.net is a good reference site for all things AR. It's not huge and overwhelming, and the members are people who use ARs heavily.
I'd stick with 5.56mm for your first AR. And, I'd make a .22LR conversion kit or dedicated upper the next caliber I got for it.
Oh, and Magpul PMAGS rule over all other mags.