If you're not going for a full track build, is the quality there in the "fixed" and "single speed" bikes? It seems like a lot I'm seeing appear to be pretty mediocre quality.

It is up to you, and what you're comfortable with. But, it shouldn't be too bad to build it from the ground up, getting your preferred handlebars, brakes, rims, spokes, hubs, etc.

And a good experience building it too... to make YOUR CUSTOM BIKE.