The Staton sounds like a nice way to go. Drop it on and you've got a solid, trued freewheel chain drive. You'd need the left pitch freewheel and a sprocket, but it looks like they've got those too.
But, if you're going to stick with the Fixie, and it is a flip-flop hub like I'm thinking, you're going to need it to be fixed on the drive side. A regular freewheel on the left would be spinning the wrong way. Then you've just got the issue of it torquing itself off the threads, which would ruin your day pretty quick at any speed. I'm not sure what the best way to fix that would be. A couple of welds would be fine, but the whole idea here is quickly interchangeable drive gears... that Staton hub is sounding nicer and nicer for a build like this. Keep us posted, it'll be interesting to see how this build turns out!