FBFs are easy to build, but finicky, and not very useful unless you have a large biological load, such as a wholesaling facility. I tried my hand at them, using both store-bought and DIY, and the DIY is probably better. It's a modification of a standard design, and I can tell you more if you want. The parts consist of 3" clear PVC, an end cap on the botton, and a threded endcap/seat at the top, plus some threaded 1/2" PVC conectors for hookup. Hell, I'll even sell you a chunk of 3" PVC cheap.
Honestly, though, a hobbyist should not need that much biofiltration unless they are way overloaded or have really dirty fish. If your NH3 and NO2 levels are already 0, which they should be, you don't need it. It just becomes another thing to fiddle with. Plus, it will go anaerobic and die if your pumps stops for a prolonged period.
But to answer your questions:
The medium is sand, and I found that the stuff sold for raindow lifeguard filters is far better than the silica normally used. It is lighter and fluidizes much more easily.
If you use something like a Hagen quikfilter as a prefilter, then you won't have too much problem with clogging.
I would also strongly recommend ball valves on the input and output. First, to regulate flow to prevent your sand from blowing into the tank. Second, when it's shut off for maintenance, shutting the valves first makes it much easier to restart.
