X2 on no plumbing in exterior walls if possible and 2 x 6 framing for walls with main drain line in it.
As for price, sounds good to me, my 28 x 42 to that stage is going to be about $60,000 but I have 14' shop walls, 2 x 6 framing on the first floor, a full second floor with 9' walls, a 300...