Mac Gaming Mods

Ok, thank you for your replies. I will buy the game and see what will work (or not). I'm not planning on hosting my own server. I will only join servers, so I should be ok then.

I don't own the game yet but I would like to know (before I buy it) if the mods, like zombies mods on the pc, will also work on my mac? Or do I need to convert/download them in some way.
If I join a server will it automaticly download the mod?