I wouldn’t put too much weight on portability. It sounds valuable, but the precise conditions can determine whether it is useful when you actually move. Early repayment is easier to model: ask each lender for the cost of selling or refinancing after a few plausible dates, then compare those...
I slightly disagree with focusing on whether 3% is polite. The wording will not rescue a price the seller dislikes, and a motivated seller will usually counter rather than take offence. The important part is defining your exposure. If valuation comes in low, are you willing to fund any gap? If...