Barry, the condition on your boat appears to be caused by the application of many, many coats of bottom paint through the years. The cracks in the bottom paint look much like the cracks in the bottom of a dried-up lakebed. When it gets that way, the only real remedy is to strip it off and start over. The more layers of paint you put on, the more it cracks and peels. I've stripped two boats now.
Ideally, all the old antifouling should come off. It would be nice if you could take off only the old antifouling, but leave the barrier coat. But, when I chemically stripped my first boat, the chemical stripper removed the barrier coat along with the antifouling. If you want to prep the boat for serious racing, you should strip everything, however much work it requires. If you want to prep it for cruising and club racing, I would suggest you keep sanding the antifouling until you can't stand the thought of doing it another day. By that time, you should have it smooth enough for cruising and club racing. Then I'd put a couple coats of good ablative antifouling on it and go sailing. Each year I'd put another coat or two on it, depending on whether it's hauled during the off season.
IMHO, wet sanding, by hand, with a medium grit sanding disk removes antifouling faster than using an electric sander.