That gauge number refers to the spoke hole diameter. If I was going to build up a whole wheel, I'd start with a name brand hub, especially since it's your brakes as well. Also, you can get parts if you need to rebuild it. Chain tension is no big deal, as 128 said. Spacing you can deal with when you pick a hub.

Lots of people on here have coaster rigs as bar bikes, they're pretty fun for that purpose. There's not any huge reason not to do it.

As for three speeds, I was really close to building up a 3 speed coaster last year, but I kept hearing bad things about the s3c hub. I'm also still trying to suss out the various arguments about efficiency, as well as gearing. I wanted it for a low-stress commuter that didn't suck in the wind (I commute on the chicago lakefront path), but I wasn't sure it wouldn't just be a goldilocks bike: one gear to short, one too tall, and only one just right.