Cash, one easy payment.
But seriously, cash, one easy payment.
I suspect you will have trouble directly financing something custom built of that age with anything other than a personal loan, home equity, or something equally creative. Either that or you end up paying an exorbitant interest rate from a non standard lender, unless you have an outstanding relationship with your local banker who is willing to get creative because you are an excellent and long standing customer of the bank.
Something late model, or from an established builder will be a whole lot easier to finance as they will have price guides to go by, or at least can see what identical units are selling for to establish a value.
On the plus side, the harder a unit like that is to value and finance, that means less buyers are able to purchase it, which should make it easier to negotiate the price. Basically a buyer's market if you are the only buyer with the cash.
Welcome, and good luck.