There is no doubt you can buy the stuff cheaper on the net. I think that kind of tweaks some of the stereo shop owners though and might hit you up for more money on the install. I could be wrong though.
As for the batteries, it really depends on what stuff you add in and how many amps you are going to be drawing. I went with the genny set up cuz I am pulling a lot and only use one battery for the stereo. It is working out great for me.
buying from the installer always works better, I think,if theres a issue they will handle it.So you run all stereo stuff off one battery with a gen to keep that one battery charged.Where does the gen exhaust go?