Best Hull type?
Just out of curiosity, what material are you considering for your double hulls? As I began to think about how a double hulled boat of that size might be constructed I realized that with most materials you are describing a next to impossible task to actually produce an water tight inner liner that would not result in all kinds of long term maintenance problems or which would not be too heavy for a boat of this size.
As I began to think about how I would go about designing a boat of this size and type, my first thought was that the cheapest, quickest, easiest way to produce the strongest hull for its weight for a one off boat would be a glass over wood boat. But as I began to think about how to create a watertight inner skin, I could not figure out how you would reliably create individual watertight compartments between the skins. I also became concerned about the affects of tarpping the inevitable moisture between two hulls and became concerned about the durability of this.
Steel would be way too heavy for a boat of this size and adding a second skin would only make the problem worse. Of course you also need to be able to get to the interior surfaces of a steel boat for maintenance reasons and sealed interior skin would prevent that.
Fiberglass might work but there are issues there as well. Obviously it would be a little more labor intensive than building a wooden or steel hulled boat from scratch. I suppose that you could build a glass boat with a very thick core (perhaps 1 1/2" or 2") and a full thickness exterior skin. The core would help absorb any shocks and prevent the inner skin from being breeched. If properly bonded the core would limit the flooded are to the damaged area of the core only.
If I had the goal of creating an "unsinkable boat" of that size I think that I would stick with a single hull but try to construct a series of water tight compartments within the layout of the boat.
Given your lack of concern for the sailing ability of the vessel you are contemplating, it probably does make sense that your primary focus is on what happens when you run aground or hit something ;^).
Anyway, good luck,