Fixed gear bikes can be either retired track bikes or a multi-speed bike that was converted to a single speed. If you're going to convert an old road bike, chose one with horizontal rather than vertical dropouts, it'll make adjusting chain tension easier. With a few exceptions, hybrids are a lower cost bike that are a true compromise between road and mountain bikes, value and performance. What to look for. If you're really lucky, you can find a good used American frame.
Steel is reliable, reasonably light weight, easy to align or repair, and offers excellent value.
In the early 90's, Specialized made a steel frame called the Allez. If you have ever seen the bikes that we have in our house , you'll see that I love this frame, and if you look hard, you can find one for a song. Parts is parts. Basically, parts is parts. Some people will swear up and down that Campy is better than Shimano, others will say that Campy isn't innovative and thus Shimano is better.