An eccentric hub isn't a bad idea, and the hubs aren't that much compared to hubs a lot of us ride. A set of White Industries hubs is still less than Phil Woods or something like that.

When I converted my road bike I used a wheelset built around an eccentric hub. The entire conversion cost less than what I got for selling my road group. Just do a search from my username and I had a lot of information about this.

But I used a road double crank with the chainring on the outside of the spider. My chainline was pretty close with a cog and it was right on with a freewheel.