its a MASSIVE question to answer as there are some many variables, I would suggest you visit your local clubs and have a look what other people are using with similar boats, its such a personal preference brushed and brushless, personally I'm brushless and lipos, does not suit all though, it depends on what you ultimately want, long run times at moderate speed, ballistic speed short run times etc, I am currently rebuilding a 4 foot aerokits Fireboat (build log is in naval section) its twin screw, brushless, run time will be 20 mins, 25mph, suits me, but others may want 2 hrs run time and 10mph,
the only thing I can advise at this stage, you mention the props as though that decision has been made, to me that seems to be building the boat backwards, this is just my opinion, I would suggest sort out your motors/esc's/batteries, and get a suitable prop to that setup, buy some cheap props at a quid a go, when you have the set up you want, give someone like propshop your setup they will then make scale props to suit, might be the cheaper option to do it this way