If you were considering a Honwave 3.2/Honda 10hp package new think about the 15hp. It weighs the same as the 10hp and for just £190 extra you have 50% extra power which would be useful and greatly help resale.
Honwave T32IE2 & 10hp - 15hp Honda outboard
If you went up to the 3.8 Honwave you would have loads of extra room but really need to go to a 15hp or ideally 20hp.
>>>have a Subaru Forester ( an estate ) with roof box, there's two adults and one 4yr old...
I think you'll be fine for space then. We have used two different estates and within the estate area (seats not folded) we get in...
3.6-3.8m SIB in bag, 15-20hp outboard, 25l tank, 12l spare tank, transom wheels, footpump, elec pump, 3 dry bags of kit, 4 lifejackets, wet weather clothing for 4 etc etc.