As a big fan of RDBMS, I would say stay away from MySQL, it can be convenient but it's generally crap. (Citation: ALLOW_INVALID_DATES) If I had to use an open-source DB it would be Postgres all the way.
Programming language? For you, I'd suggest Java based on your familiarity with C#. That would open you up to doing both web and "fat client" development and as a bonus your fat clients would be cross-platform.