Promags pretty much suck... that being said you can often rehab them into usable range mags. The bodies and followers are OK it is the springs that suck. Get Wolff springs and they will often run. I would not use them for anything but range work.
The folded carbon steel slide is sought after because it is no longer used and because it is lighter than the milled stainless. Guys who cut their teeth on the older P series guns prefer the balance of the older slides. These are the ones i prefer. The stainless feels top heavy to me.
When properly cared for they will last a lifetime. As to the trigger bar spring I would look at it again. Make sure it is installed properly and put the dremel down..... more guns have been ruined by those contraptions then helped IMHO.