things to look for in old road frames to convert to fixed gear use ( not in any kind of order or hierarchy)
a) steel frames, with cro-moly, or other steel alloys being strong AND light, aluminum will certainly work, but it is SO stiff, you usually need a cro-mo, or carbon fork so you don't get the crap beat out of you on the rough roads we all have to ride on.
b) standard ISO BB threading
c) forged horizontal rear dropouts, the longer the better (campagnolos are REALLY long) but again if you have cash (eccentric hub, or BB) or creative ingenuity AND cash, you can make vertical dropouts work just fine, keeping in mind that if you use a chain tensioner, your NOT going to ride it fixed, but single speed freewheels will work
d) brazed lugs (totally esthetic, you can get GREAT quality steel welded frames, but the lugs USUALLY indicate higher quality tubing than say, plumbers pipe (like that murray for instance!
)
e) stem mounted shifters (so there aren't any shifter bosses on the down tube)
again esthetics, but it looks cleaner without the holes on the downtubes.
really you can make a fixed gear bike out of almost anything, but if you stick with the above list, at least you shouldn't run into any roadblocks along your conversion/restoration journey, and I also would recommend waiting for a better frame to come along.
I see older schwinn's, miyata's, nishiki's and centurions on CL and on fleabay ALL the time, and usually for about $50 or less.