Having owned and restored several wooden boats, I have found that a wooden boat that is in good condition requires about the same amount of maintenance as an older fiberglass boat. The big difference is that a wooden boat sets the schedule for when things need to be done and if you don''t keep up the problems can grow rapidly.
To the specifics of the boat in question, typically wooden boats are not fiberglassed over unless they are either plywood, or because they leak. There are certainly some exceptions to this. (Strip planked boats were often glassed in the 1960''s for example) Since this is a plank on frame boat I can assume that the boat had cronic leaking problems and glassing over was someone''s solution. When wooden boats leak there are typically a variety of causes, but for a plank on frame boat built in 1959 you would expect the leaks to be the result of a need for recaulking and at least some refastening.
And that gets you to the heart of the problem. If the glassing job was done well, it will last for a long time. Done well means that all paint was stripped from the planking, the boat was refastened,epoxy resin was used for the laminations and a minimum of two or three laminations of glass and resin were applied (more is better except from a sailing standpoint-see below). Because wood will continue to expand and contract, the superior bond of epoxy is necessary and the extra tensile strength of mulitple laminations is also necessary.
The problem is that most wooden boats were glassed using less expensive polyester resin and don''t seem to have the proper preparation that it takes to create a good bond. Similarly, the refastening job is often ignored to save time and money, and so the wood below the glass continues to work more than is acceptable. I have seen the glassing job last less than 10 years (and one job lst less than two years)and have all of the worst problems of both glass and wooden constuction when the glassing job is not done correctly.
The other problems that you have noted are pretty serious and are not typical for a wooden boat in good shape. The rot at the hood ends of the frames occurs more than is ideal on woodenboats and can be structural is it is occuring on more than one frame. While you may only see the rot in one frame more typically if one is rotten, many more have rot as well. It sounds like you are describing frames that are pocketed into the side of the keel. This is generally considered stronger and a higher quality constuction technique but it also limits the life of the boat because eventually water gets into the pockets and rots out the ends of the frames. This rot starts at the end grain and moves up the frame where the frame bears against the planking. The three sides of the frame that are exposed to air can dry out and so they stay intact and rot free. The core of the frame invisibly rots out (a condition called troughing). If you only had one bad frame, you can dry it out, inject it wih epoxy and go on in life, but if troughing is present in multiple frames then something more dramatic should be done because the garboard strake to keel seam is a very high stressed area of the boat. In the worst case, where the rot has spead into the keel itself, or you ahev a lot of bad frames, the garboard strakes should be removed, the keel should be consolidated or a dutchman added and the frames scarfed sistered and repocketed, you are talking about a lot of work, expecially with the glass over.
The split frames are a separate issue. Frames split for a lot of reasons. Small splits parallel to the grain may only be drying checks and are no big deal. Bigger vertical splits generally occur when a boat has been badly refastened, refastened once too often, or has troughing in the frame that has now been dried out. None of that is good news, but without seeing the boat it is hard to advise you further.
One final point, while small schooners have a certain charm, a schooner rig really does not make sense in boats under 40 or more feet in length. It can be done, but it is not all that likely to sail well. The one exception to that rule that I know of was the small Tancock Whalers but, for a working watercraft, they were quite light with easily driven hulls.
If you added the extra and unplanned weight for a proper hull glassing job, I would not expect this boat sail very well. (In the worst case, internal ballast was often removed from wooden boats when they were glassed which when combined with weight of the glassed over topsides dramatically raised the center of gravity of these boats and reduced stability)
That''s how I see it, but I have not seen the boat and so I am only speculating based on my past experience and your loose description. If you are genuinely interested, do a sail trial and if you like how she sails, get a marine surveyor who really knows wooden boats.