Through the middle of the country runs the Balkan Mountains (Stara Planina).
Plan on going over them at some point; we managed a relatively simple pass on the Black Sea coast of only a few hundred metres. We paid for it with head-wind. Road quality varies greatly,
but is generally decent. Traffic was never a problem, and was considerate. Road maps were troublesome though, since it seems Bulgaria's resurgence is producing many a new road:
expect to find new roads where you least expect them, even expect them to be renumbered.