I wouldn’t choose a contingency percentage first. A large reserve does not cure a vague scope, and it can hide an unrealistically low bid. Have each contractor separate fixed work, allowances and excluded work, then explain what would trigger an extra charge. For concealed plumbing or damaged...
Is 15 years the entire repayment term, or only the fixed-rate period? If it resets afterward, what term and balance would remain? That matters more than portability to me, because a comfortable payment now could become uncomfortable after the reset. I’d ask each lender for the monthly payment...