I'm thinking about building one of these fine British SMGs. I was originally thinking about making an AR 9, but that seemed too boring for me. I'm now thinking of a Sterling or Sten Carbine.

They both seem pretty straight forward to make semi autos. The Sten has that WW2 cool factor, but the Sterling was in Star Wars.

Anyone build either, or have any advice?

I think the only huge hurdle to jump is making a Sterling featureless without a pistol grip and fixed stock. The Sten already is featureless with the Candian Stock. Also maybe 922r compliance.

I can get 10 round Sten mags by asking some family members in free states to just pin them.

9mm AR would be my suggestion, but I don't follow milk bag politics.  

I did a STEN pistol earlier this year and it is a hoot.  I bought a kit from Indianapolis Ordnance since it was my first STEN build.  

Much to my surprise, it shot to point of aim, and never jammed (despite the fact that I left the extractor out when I put it together.)  It ran cheap Tula 9mm steel cased ammo without a hiccup.