Necramech mods feel far too rare. I'd suggest that Loid sells them for standing, increase the drop chances to 25-33% (from 10%) and/or make the wandering mech neutral instead of allied, so those who wish could attempt to take it out without needing to do iso-vaults repeatedly.
I'd say build the necramech and rank it up with a archgun that you already have ranked? I'd assume they grant mastery since they are ranked up like everything else. The kitguns (and by extension the saws) only give mastery for their barrel (or strike for saws) only.