I'd probably go with 9 ft RIB on that boat. Single
floor to save weight. 10HP is plenty on a 9 ft. For two people you might get by with a lighter engine. Just depends on how much you want to plane fast and how much you intend to carry.
The other option to save weight is go with traditional ply floor inflatable. It's not like you have to disassemble it with davits.
Take the motor
off when using the davits. So you might want an engine crane for that. (Garhauer)
Achilles is one of the few brands that have been consistently good for many years.
You don't want a lot of weight slung off the back with a 31 footer.
I had two AB RIB's. One 9 ft and one 10 ft. Both heavy double floor. The 15HP Yamaha was too much HP for the 9 footer. Although I used it.
Re-reading your post I se you have minimal space forward of the mast
for ocean crossing
.... I lean toward the conventional Ply floor(or similar) inflatable if you are going to put it on deck and cross oceans.
The old Achilles are bullet proof, you might find a good Hypalon used one and save some bucks. Not like you cant get a new one down island in a couple years if you decide to