If you are needing Sterling mags, and have an FFL, then get a hold of Century Arms as they were having them on sale for $10. Mine came with varying amounts of finish and only a couple had some surface rust of an inch or two in size, few had some spotted surface rust, but none of the mag bodies or feed lips were dented and the followers went up and down freely. One of the mags was an L1A2 mag and the rest were the standard L2A3. I used to have a bunch of Sten mags because they were so cheap but now I have twice as many of the more superior Sterling mags due to this sale at Century.
I will have to check out Gunco.net for your build and when you're finished with your build you should think about putting it on www.weaponsguild.com as well. I can't believe I haven't seen this thread before now and look forward to checking it out as I really like guns that are converted to 9mm.