yes i've done a few golf carts. no idea why you want to put anything other than subs under the seat?
speakers up front are a given, obviously a marine HU as well, usually in the pockets. marine coaxial, nothing fancy, high efficiency is best (90dB+). something that can get wet.
some marine 8's under the seat are doable, but don't sacrifice enclosure volume for cone area or an amplifier location. if you can fit subs under the seat, a small amp is no problem.
I've actually been surprised by Powerbass marine speakers. they were better than Clarion and Alpine. A small full-range Class-D marine amp would fit under the seat, but we'd need to see pics and know the dimensions to be sure. Alpine has one.
A 4 channel amp is all you need. two channels to the speakers, bridge the other two to the sub(s).
note that you will drain one battery more than the others, which will shorten it's life and the overall range.