Working out how much to borrow
Start from the monthly repayment you can carry, not the largest amount a lender might offer.
Look at an ordinary month first. Take what comes in, set aside the bills that always land, and see what's left after food and fuel. The repayment has to fit inside that gap with room to spare, because few months stay ordinary for long.
Then work back to an amount. Covering part of the cost from savings, if you have them, makes the repayment lighter. And if a lender offers more than you asked for, that's no reason to borrow it.