Thanks horndude, that sounds like some good options to explore. Something else I was thinking of was whether plain old 135mm freehub wheels come with spacers on the axle ? Looking at
this stuff on Park Tool's website it looks there is a small 1mm or so spacer on each end of the axle, I could probably just remove those and have a sweet fit in my frame. That way I'd be able to just get a cheap freehub wheel off ebay and throw on a single sprocket. Any obvious problems with going that route ?