There is an interesting parallel to your story and our own. We decided to become sailors when I was 50 and my dream was to cruise around Florida after i retired. I took a sailing class at Lighthouse Landing at Kentucky Lake and bought a new Hunter 260. We sailed it for 5 years and figured out what we both wanted in a larger sailboat and in 2003 bought a new hunter 356 and fitted it out with everything we dreamed of. It was, and still is, one of the most high tech and live able boats on the lake with a generator, satellite TV, and all the electronic gadgets. We keep our HVAC unit on 24/7 even when out sailing so our salon is always a comfortable temperature summer or winter. We have a full enclosure and sail 9 months a year. We spend an average of 75 days a year on board, so we get our money's worth of use. We look at our boat as a vacation home we can move to where we want to.
We are 66 now and plan on soon taking our boat to Florida for the winter. We plan on leaving it in Charlotte Harbor, where we have lots of friends with condos and houses, and go back and forth between Tennessee and Florida, living on it and cruising the West Florida coast and Keys while there and enjoying being with our Tennessee friends that winter in PGI. We have done a lot of sailing there on rentals and with friends, generally making an annual sail from Charlotte Harbor to Key West each winter for a number of years. We will bring it back to Kentucky Lake in the spring for the summer so we can continue what we do now.
Other than purchase, it's not too expensive to maintain, but it's relative to what you are used to spending. Slip fees in Kentucky Lake are a couple of thousand per year, in Florida would be at least three times that, bottom paint and just general maintenance and insurance add more cost. Get at least a 35 or larger boat. You end up with a lot of stuff and will need the room. Make a list of what you want and then buy a boat with that on it or add to what you buy so you end up meeting your desires. If you are not happy with your amenities, you won't spend as much time on the boat as you would if it meets your needs.
Don't let size worry you about sailing the boat. With the Hunter set up with in mast furling and all the lines in the cockpit, an anchor windless, auto pilot and other aids, you can single hand a Hunter 356 or 456 easier than your 240. it's how you equip for single handing that dictates how difficult it is. My 260 was a lot harder to sail and dock than my 356 is because it was barebones. When you cruise, you motor a lot if you are on a schedule, but a sailboat is a very efficient cruising boat under power and you have the option of motoring or sailing at your choice. We do a lot of both. With my generator running, my fuel burn with both generator and engine is .64 gallons per hour at 6.2 knots. The generator is about .25 gallons per hour, or 6 gallons in 24 hours.
Sailgunner is right about size. Look at a 40. They are a good cruising size boat and if you have guests, you need at least a 40 if they overnight with you. We draw the line at guests during the day only in our 356, we get them a rental cottage at night. We would still do that if we had a 50!
Enjoy yourself and good luck in finding your dream.