I agree that getting practical on the water experience is key when it comes to
learning how to sail and general seamanship. Get out in the water as much as possible.
I don’t agree that you want a
race boat like a 420. An optimist is almost too small for your daughter and would be impossibly small for the two of you. A 420 is unstable and relatively complicated for your needs. You’ll be better off with a
family day sailor, around 17’ or so with a nice sized
cockpit. I don’t know what are prevalent in Greece but something like a Flying Scot would be perfect.
There is a lot you can start to study now like
navigation and
weather. Yes on the water experience will provide necessary context for all that knowledge but being familiar with basic coastal navigation before you set out will be helpful.
Get in shape. Life on a boat benefits and at times requires a basic level of fitness and anything more than that helps. There are a lot of motions and things that you’ll be asking your body to do and if you’re not in shape, at the age of 60 you may find yourself
learning by way of injuring yourself which is never fun. Flexibility is also important when you start cramming yourself into lockers or contorting to fix stuff.
I would be careful about selling your flat to buy a boat and if possible explore other ways of
financing it. A boat is a depreciating asset. If you can rent out the flat you’ll maintain that capital instead of watching it slowly vanish as the value of your boat slides into oblivion. And you’ll likely move ashore again someday.