Daniel Ray wrote:Cob Cottage Company is based in Coquille Oregon which gets an annual 55" of rainfall each year and their buildings are in perfect condition. Cob is more than suited for wet weather you just have to protect them adequately. Get the "hand sculpted house" by Ianto Evans or "The Cob Builders Handbook" by Becky Bee; i've put both links below. You need to have a good "hat and boots" which means build up your water impermeable stemwall well above the ground and make nice big overhangs for your roof, think 2-3 feet.
Cob will be too heavy for a platform build house, you need a rubble trench foundation and that will definitely help with drainage. Again, I highly recommend the books mentioned as they outline all the questions you might possibly have about cob. 3 months is short, but doable if your design is 200-400sq feet. Get some friends together for a building party. I would recommend building with strawbales just for speed. You could stack all your walls in a week. The gravel for rubble trench can be anything, sharp gravel needs much more compaction than round drain rock.
Make a few test cobs--like brick size--with the soil you have and see how it fairs to dropping on the ground. As for tons, do the math for volume with your walls. 7' average wall height at 2' thick with a run of 70' of wall is 980 cubic feet of material. That is about 35 yards of material at 1 ton each. This answer was brought to you by the really rough estimate organization, where we estimate things that cannot be properly determined from my work computer.