If I were you I'd look at the following:
1) 500 watts for 3 subs is a little light (depending on the sub model your using).
2) A 75 X 4 amp will not give 3 pairs of component speakers that much power
3) If the subs are going in the rear seat base (great location IMHO), make sure they incorporate proper sized and constructed subwoofer boxes into the base. My suggestion is to have them build the enclosure out of 3/4" marine plywood, glassed, resined, etc. MDF takes a beating in a boating environment.
Not sure on the pricing. Sounds rich on the equipment side.