I was thinking of trying that out with a flipflop single speed wheelset I had. Though I'd have to get smaller lock nuts as the freewheel didn't want to slide over them.

Last time I checked you could get prebuilt single speed wheels on Ebay for a decent price with flipflop hubs and deep v rims. Do a search for Eighthinch wheelsets. You can pick them up for 110 shipped. Even comes with a cog and lock ring which would normally set you back 20-30 bucks. I used to own a set and not to mention I lived a few miles from the shop that sells them.

Pick up stuff like wheels and bigger pricier stuff first and upgrade your frame later and just swap the old stuff back on and sell that bike after you get something better.