I would buy it if:
The rims are something decent
The wheels were laced by someone who knew WTF they were doing
The tires are good to go
The saddle doesn't suck
The pedals are not the old crap, or at least have been rebuilt
Basically saying $250 would be the most I would pay for a conversion unless the wheels were brand new or the frame was something sexy. Otherwise, it would need to be a bunch of good parts and done well. Also, what era is the bike? Mid 80's? Any tubing stickers?