Lots of great questions,
@ClayWelder .
It doesn't sound like you are doing anything super crazy, so the basic forward arms would work fine for you and you'd save a few bucks. BUT...........The Atlas Pro arms are definitely the best option out there. Super beefy. You'll never bend or break one....and if you do somehow....you'll have way bigger issues to worry about. Lol!
With both arms you can run up to a 31" tire. That's before adding spacers. I'm not quite sure if adding wheel spacers will effect the max tire size...but its possible because you're wider, thus pushing the wheel further into the wheel well. If you're sticking with a 28" you'll never have to worry about that. And even if you do go up to a 31" with wheel spacers and find some rubbing....it will be very minimal, if at all, and only in the most extreme situations.
Its a good time to go through all the bushings, bearings, and ball joints while doing this so you can knock it all out at once with good fresh parts.
For axles, definitely go with the Rhino 2.0. Easily the best bang for the buck out there. Can't beat them.
Ball joints....our HD ball joints are awesome. Keller's are awesome too. Either way you go, you can't lose.
For the frame stiffener, I highly recommend it. The Pioneer frame tabs are notoriously weak and flimsy. That should suffice, and you wouldn't have to worry about the weld in gusset kit I wouldn't think.
For wheel spacers, I'd always go wider unless you are limited to a certain width by trails or by your trailer. I've ran wheel spacers on every machine I've had.
I think I answered everything you asked. Pretty sure. If I missed something, I'm always here checking in. Feel free to PM me if you ever need anything or if you are going to pull the trigger and want the forum member discount code.
Sorry for the long-winded answer, but I think I have it all covered.