If you want to do this on the cheap, have you considered just building a single speed? That way you wouldn't have to replace the rear hub at all, could use the old derailuer as a chain tensioner (if you can't find a magic gear) and would probably only need to spring for: new chainring bolts (or some washers to space the old ones out) and spacers for the rear cassette (and maybe a cog).
You could then spend some time saving/searching up for a cheap wheel while having a very clear idea of what was needed to make this bike fixed.
If you want to do this on the cheap, have you considered just building a single speed? That way you wouldn't have to replace the rear hub at all, could use the old derailuer as a chain tensioner (if you can't find a magic gear) and would probably only need to spring for: new chainring bolts (or some washers to space the old ones out) and spacers for the rear cassette (and maybe a cog).
You could then spend some time saving/searching up for a cheap wheel while having a very clear idea of what was needed to make this bike fixed.
Just a thought.