Well, I own and love my .458 SOCOM you can use it for any game on the NA continent bar none.
1. Your lower will not take a beating, so use whatever you like! I recommend going to the Tromix website and purchase a complete upper. Tony was one of the developers of the round and is a great help if you have any questions, you can also get .450 Bushmaster uppers and 375 SOCOM uppers if you want.
2. 458 SOCOM vs 450 Bushmaster the main plus to the Bushmaster is that it is a straight wall cartridge. A little easier to reload...and you WILL want to reload so you can afford to shoot ANY big bore and this will be no exception. The 458 uses standard GI Mags, you need proprietary for the 450 Bushmaster (ergo mags are cheaper for the 458). Bullets I find a much larger selection for the 458 since it is the same bullets you use in a 45-70 if you shoot that caliber already, the 450 uses .452 which is basically 45LC/454Casull etc. so honestly it all depends on what you currently have....if you reload...if you are buying factory ammo, then this aspect does not matter so much, but save your brass either way! Other than that, it is a personal preference.
3. If you scope this rifle you will need GOOD Hvy Duty 3 screw rings!!! Or you will beat a scope to death! It isn't that the recoil is all that bad, but that the AR platform is inherently light and while it won't punish your shoulder if there is ANY movement in that scope you will be purchasing a new scope before long if you don't catch the fact it moved! Even if the scope does NOT move I would hesitate to put any electronic heavy scope, like a night vision scope. I put an ATI scope on mine and it lasted 3 rounds. Got it back from the company after they "fixed" it and it lasted 6 more rounds and then that was that!
4. Range and accuracy....totally dependent on the shooter, I load my 350gr soft points up so the are moving at 1800fps+ so if you shoot the rifle well 200+ yard shots are a piece of cake, and yes, the bullet will be slower at that range.....but it IS a heavy bullet, it will still kill most anything on the continent when it hits if you do your part! As for accuracy, well that is going to be up to you, the running joke is if you want to get a 1/2" group at 100yds just fire one round! But if I do my part with my Leupold 1-4x20 on mine I can do sub-moa.
If you have any specific questions or, if you find your way near Rock Hill SC let me know and you can try mine out!
Here is mine with a 16"bbl...the entire rifle is made by Rock River Arms: