As for the size I think that is on the small side, if you live by an Ikea store you could go in and see what 350 SF looks like. You as well can ad on to both styles of homes your talking about. Depending where you live your foundation could be free if you live in a place where you can gather field stones or flat mountain rocks that fall to the side of the road, that’s what the person did I helped. She sourced a lot of stuff from junk piles, windows, doors, got wood cast offs from lumber mills.
I would feel comfortable in 475 personally. I have helped on a cob house, it is very hard work, you would need to make several
cob bricks of different mixture to see what one holds up the best. Are you keeping to one floor? as far as the outside goes she used Lime plaster, mixing it you need to be very careful!
