[dg7feq]

Well, not so easy to answer. For voice communication all networks here are equally good, if you want to be secured of bad surprises just stick with the carriers themself and not 3rd party SIM-cards. As you said you are not placing many calls this will not make much of a difference.
As Bulgaria is in the regulated EU-rate any german card will charge you 9ct/min for inbound calls (rounded by the second).

Data is a completely different story, there i am not the expert, maybe some other members of the community have some smart suggestions.

Chris