be sure to get a loading manual. It wont have 458 socom recipes in it but it will have all of the fundamental process step for reloading and is a must have. Also you will need a tumbler to clean your brass. I recomend Imperial sizing wax for resizing lube on the socom, the brass needs a good full lube job to be sure you dont get a case stuck in the resizing die. You will also need some other basic case prep tools for trimming to length, chamfering, beburring, etc.
As far as supplies, you wont find many bullets in bulk quantities for the 458 caliber other than cast lead and maybe some basic hollow points. As far as where to get stuff I usually use Midway, but there are many choices online if you can't find stuff local. Cabellas, cheaper than dirt, etc. Don't worry about getting everything asap cuz you will be waiting a while for that RRA. The industry is slammed right now and I have heard manufacturers are having a very difficult time getting 458 barrel blanks. I ordered mine before the panic and still waited over 6 months.
Good luck with everything and be sure to get plenty familiar with the reloading process with your buddy on some more common rounds before you start working on socom loads.