I did this last year to an old peugeot which had been hanging up in my dads garage.
There are some photos on flickr[/url].
VeloSolo and lfgss[/url] were incredibly useful as was the sheldon brown site.
Mine had semi horizontal dropouts which were perfect. I put a fixed cog on the back and used threadlock to stop it unscrewing. Not had any probs yet. A bmx freewheel should be fine too.
I did have to redish the wheel to accommodate the new chainline. Not as hard as it sounds. If you do it one spoke at a time. tighten spoke one half turn, loosen the next one half turn and repeat round the rim to pull the it over.
I learned a lot during the process.
HTH