First the AGD Intellifeed can only help BPS if you learn how to shoot it right, I myself prefer it over the single trigger, and Benchmark. The bodies are made of Stainless steel or Aluminum, they do not rust. If you have original stainless bodies you need the twist lock barrels, almost all others take cocker threaded barrels to my knowledge.
You have already bought another mag? Do you have any pictures to post of it? They are too much fun to play around with, but you will find that once you get one configured the way you want you should never "need" a back up marker. My original Automag from 1994 still works great and I am the 2nd owner. They are built like tanks. My suggestion to you is save some money and go play first.