If BF has $250k, he probably has his head on straight, but given your larger net worth, I would insist on a prenumptual agreement prior to getting married. To the original question, if you are going to need the space, and buying is the way to get it, buy it before you retire. Once earned income drops to $0, it can become exceedingly diffcult to borrow money, even if you do have $1 million in the bank.
As far as the rental question goes, how much is your studio worth? Are taxes and insurance included in the $540/month fee? Depending on what you can get for it, you might be better off selling and taking on a smaller mortgage.