So, if you buy a used one. If it's newer (past 2-3 years), there is a good chance that it will have a freehub. If it's older, chances are it will be a fixed hub. If it's really old, then it's guaranteed to be a fixed hub.
If it's really old and has seen lots of use, change out the chain and maybe the crank arm + chainring combo (they are forged together). They get "shark-toothed" like any other chainring, even though they are steel. Make sure to buy 2 single speed chains. One might not be long enough.
You can order all of the parts from CycleOps.