I've been toying with this idea too. I considered the 'old touring bike' option but I have an old GT mtb frame that I think should be used. It has vertical dropouts so I think I'm gonna set it up singlespeed with a tensioner for now but I intend to get it fixed in the near future. I have considered trying to find the 'magic gear' to fix it on the cheap, but I'm still not convinced its safe, what with chain stretch etc.....