Three blokes you are going to need 3.4/3.5m absolute minimum and a 25hp. Did you check the sticky? (Just to avoid repeating all that is in there
). If there are three kids and a woman + you then a SR4 would be useless really and you would need a bigger RIB ideally if you did solve the trailer/storage issue. It depends on the kids ages too as to how safe they will be/confident you will be in them holding on for dear life and if they can sit on the tubes safely.
I think the Futura like virtually all SIBs has a pretty flat bottom so in any sort of chop it will slam and your kids/wife will not be happy. Aerotec/25 would be better but a SIB is not a RIB and they are light and jumpy and will not cut through waves/chop like a RIB but a BAT will do it better than other SIB's. Regardless with kids on board you have to choose your days/conditions carefully anyway. Buy secondhand all round as advised. A package is nearly always cheaper, comes with goodies and usually a better risk re' engine history.