>>2001544i would look for a nice old hybrid or tourer with beat up components to convert. In the correct size, with paint you like.
the critical thing is just getting horizontal dropouts
I don't see why you need 120 spacing for single speed. Going SS on just a regular cassette hub is fantastic because you have the whole freehub of spacers to move the cog around and achieve a perfect chainline with whatever crank you choose.
It doesn't look quite as clean is all, but then, you could also use a nice threaded road wheelset with a SS freewheel.
