It doesn't matter if the bank manager is familiar with it. There is no rule that grandpa can't sell his house to his grandson.
How much money the bank lends you is not up to the individual. He has a fixed calculation standard and needs to be evaluated. After the evaluation, the bank will give the house a price, and the bank has the final say.
Whether the loan can reach 400,000 or even more needs to be evaluated. I suggest you find a friend who is a local real estate agent. First, ask which bank is better to cooperate with, which bank has better loans and how much you can borrow.