Welcome to CF.
A whole lot of the success of your plan depends on your and your good lady wife's physical condition. If you're planning on tearing your wife away from her grandchildren to fulfill the dream, you're going to have a stormy upwind journey.
Perhaps if you tell us more about yourself, we will be able to offer better advice to you, but planning to start off on a circumnavigation
at the age of 65 and over, with no prior sailing experience and no mechanical experience to speak of would be a recipe for failure of the plan.
As we age, we become less resilient and if you and your good lady go out, and she becomes cold, wet, and terrified, this will not be an auspicious beginning. It MUST be fun for her, and you'll be partly responsible for that.
I am not trying to dissuade you, but even if you buy a brand new boat, you should expect to have to fix things, it's the nature of the beast. It is not like choosing and buying
an RV or caravan to explore by land. The ocean itself can be a hostile environment
for people. Do not underestimate what you're about. Jim and I are wrinklies, yes, but we've been cruising over 30 yrs. It really isn't for everyone.
If I were in your shoes, I'd buy a smaller yacht (26-30 ft,) and both of you start sailing now. See if you like doing it. Learn to fix *stuff* yourself. If you find it's fun, you're on your way. But if not, then you haven't blown your retirement
funds. The longer you wait, the harder it will be.