Me and a couple of my friends are planning on trying to make a source mod, but we can't figure out how to tell what game to use as a base. We hope to make a totally different game with it, and aren't sure how much of the base game will actually hang around. Like if we chose TF2 for the base would we be stuck with cartoony graphics and teams?
Based on my limited knowledge, your choice of base game gives you some of the core mechanics pre-programmed, along with a few textures, models, sounds, and the like. As such, it seems that if you wanted to make a linear single-player game, stick with one of the Half-Life 2 games as a base, or if you wanted to make an arena multiplayer game, stick with HL2MP, TF2 or Counter-Strike.
However, Valve also offers the Source SDK Base, which locks the engine to a specific version, and comes in singleplayer and multiplayer variants. These use base code from HL2EP1 and HL2EP2 (depending on version), but provide content from HL2 and HL2 Lost Coast. This article might be a useful starting point.
Yes, it matters. Only certain editors work with some games, and only certain textures work in some games.