Yo Tim, I think I may have interacted with you or one of your friends in the past with some gun stuff sales. Ill try to help answer most of your questions above with the limited experiences I have.
1. Are you set on using an AR platform? I strongly advise not using an AR for hunting, unless its just for pig control or unless hunting from like a blind you can walk to. But if traversing through terrain and trying to stalk animals, a bolt gun is far superior with regards of ease of use in compromised positions, safety, and simplicity in the mechanism itself (less things to go wrong). Also a bolt gun is a lot more streamlined to carry in a scabbard or hunting pack.
2. If you are concerned with swapping out barrels, go to snipers hide or longrangehunting forums, or PM me. swapping barrels on "certain" bolt guns have become 10x easier than swapping barrels on an AR platform. Just to give a quick snippet, if you went with a Tikka, remington 700, savage, or custom action, they can all have barrels that can be ordered and swapped w/ a headspace gauge and nut within 15 minutes. Custom actions can be swapped without having a barrel nut at all, its as simple as screwing in a lightbulb, no joke.
3. As far as calibers go, that is going to be a difficult one for you to decide. If you are 100% sure you don't want to reload, just stick with 308 and practice practice practice (this goes for any gun btw). Just because its a hunting gun doesn't mean you only shoot it 1x a year to "sight in" that is the worst mentality to have. Your hunting gun should be one of your most proficient guns and you should be comfortable behind it in various positions. We all owe it to the animal to make clean and ethical shots. But if you think you are going to reload, 6.5 creed can be great because at least it has some factory offerings. 7mm-08 is a great round but has many shortcomings, one being the very limited factory ammo and the fact that it can't push 7mm bullets fast enough to keep up with most 6.5 cartridges. As far as reloading supplies go, you can easily find 6.5 creed, 260 rem, 243, 6.5x47, 7mm-08 brass supplies on a plethora of sites online.
4. If you are purely reloading (or decide to). I would say forget the AR, reloading for a bolt gun will make your life much easier. And then stick with a common round like 6.5 creed or 260 rem. That way you can find a recipe on various forums that is within 1 grain of an acceptable load and load workup will probably take like 30 rounds max. Also, reloaded ammo is by far cheaper for the same quality of great match ammo, and you might not even find a match ammo or hunting ammo factory load that your gun likes.
5. As far as shooting distances, I honestly would say you should keep shots <250 yards, unless hunting bigger game. If you do plan on shooting at 400 yards, at minimum you should see if you can shoot over someones chronograph to gather velocity data and create a DOPE chart (via ballistic app online) or drop chart and windage chart so that you can make a clean shot. I hear a lot of local guys talking about how they take shots at animals at 400+ yards when hunting, but yet they can't tell you the velocity of their rounds or are using a BDC type reticle to make various "holdovers" based on what the box of ammo in front of them says or their scopes manual. The way I see it, if your gun shoots at best 1moa at Kokohead on a bench or prone, in a hunting situation you are probably a 1.5-2moa shooter. If we take a shot at 400 yards, that means at best we have ~8 inch variation. Take in consideration wind, adrenaline, unstable platforms, other factors, and that can lead to a gut shot or low shoulder shot, or skim shot. I don't know about anyone else but that is the worst feeling on earth, ya you might recover the animal, but not something to be proud of for sure. As far as stopping power, you willl not seee much difference between the above cartridges within 600 yards. But yes you are right, the 6.5 or 7mm bullets will outshine the 308 in margin of error for wind.
6. Sorry for the long post. Hope that helps answer your questions. Feel free to see what others have to say as well. I am by far no means an expert at anything, just sharing my experience between myself and brother shooting bolt guns to hunt goats and practice shooting 600-800 yards. My brother shoots a 6.5x47 lapua this year and I'm shooting a 260AI. Just to give you perspective, we were holding 1.2 mils at 820 for windage and 6.9 mils for elevation.