I think it would make a bad single speed conversion candidate (even though I happen to like freewheeling single speeds) because of the cottered cranks. To convert this you'd want a new crankset, which would necessitate a new bottom bracket..plus wheels etc..

for 100, if it were me, i'd just clean it up and ride it. I'd be willing to bet it doesn't actually need new brakes, but just new brake pads. around 10 bux a set. And some new shifter and brake cables never hurt and are under 20 bux for all 4 cables.

You can get a set of cheap replacement wheels at Niagara Cycle for under 100 bux that will be much better than whats on there....you could use 700c or 27" either will fit and the brakes should reach fine for 700c.