Toby I am in Ribs marine in Christchurch your welcome to take a look around. I have a Marlin merry fisher 695 with a single 150 yamaha.
I have most of what you want and at the cheaper end of the market. Once you move to twins you double your fuel and servicing and as mike says refuelling can then be a chore.
Ribs owners now own Rossiters and there is talk of possibly installing a petrol pump. The class rating B is for offshore, most boats are C rating but will do most things on calm seas and nursed at 12knots will do most of the rest. If your out in worse than that you have made a trip plan mistake.
If I was upgrading for Christchurch with your wish list I would go for a Barracuda 8 but you will need 60k. Where as 35k might buy you what I have. I would prefer a cat for Christchurch just because of the sand bar and draft issues in the Winter or beyond November.
If you study the commercials they prefer Cats in Christchurch as they sit square rather than listing if you run aground by accident.