Depends on what you need...
Obviously, you will need either the .map files or the .bsp-loading code from the game. From there it all depends on what you want... If you want an fps, I'd say get some recent but stable quake3 svn revision and start modding. Alternatively, you MIGHT want to start with the Tremulous code due to its admin features, but again, that depends on your preferences, and what kind of game you are making...
Actually, now that I think about it, your first step should probably be to recruite some reliable help. Also, you should ask the mapper before you use their map, as they might not like you just taking it for use in our game.
Good luck.