Congrats on your choice of the 458 socom, which given the facts that it has a bit of a shoulder and can fire both rifle and pistol style bullets that are available for it it's a mystery to me why people choose the 450 bushmaster over it (unless maybe they live in one of those weirdo straightwall hunting restriction states).
I personally feel it is worth it--expensive as it is--to invest in tromix's modified Lancer single-stack magazines because they will increase the reliability of proper feeding of all style bullets including blunt nose pistol ones. I have a pet load using Lehigh Defense extreme penetrators which shoot very well out of my 458 socom (and, not coincidentally, out of a 45-70 as well). Be prepared for a "startling" big boom when you fire it, getting a good brake for the muzzle will help overcome that and the recoil to help you get nice groups with it. As for the lower nothing special is needed, though you may want to consider something like a kinetic energy buffer and a bit of good padding for the butt of the stock, the energy generated by a stout load is comparable to a moderate 45-70 load. I found it makes a difference if I'm out testing 35 cartridges of hand-loads.