There's not really anything u need for a "raiding garrison". Salvage yard or war mill help a lot for getting your followers geared up to get some missions to get you gear. Other than that, you can pretty much get whatever you want. I made garrisons on a few of my alts for different profession buildings, but it wasn't necessary. You get 3 small building slots and a profession building takes 1 small slot. You'll probably want to get 2 profession buildings in the small slots to line up with your current professions on any character you get a garrison on.
I'm not sure i explained that right, but no, you don't need 2 separate garrisons for raiding and professions