Hi Aorta,
Buy a 150-160L Freeride board, around 80-85cm broad. I can recommend something like JP Magic Ride, Starboard carve, Fanatic Gecko... That will suit you well for now in the 10-18kn winds, allow you to learn balance on the board, learn basic tack and jibes and get into harness/straps and planing over the next two seasons.
I'd buy a 6-6.5 freeride sail for now, and later a 7.5-8 m2 sail for lighter wind planing.
I recommend to spend some more bucks on good/light equipment - makes everything easier. Don't buy pieces which don't fit together. Starting to surf is already challenging enough, no need to make it more difficult by using unsuitable equipment

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Try to buy sails/masts from the same manufacturer from similar years to ensure the rigg behaves well (different manufacturers use different mast curves). That strategy also allows you to add more sails to your quiver later on without always buying new masts. As for boards, keep away from daggerboards (not fun after 3 sessions) or anything with race, slalom, formula or wave in it. Go for freeride only.
After starting on a 160L board and good learning, you might want to buy later a 120-130L board for stronger winds. But the 160L board you can still keep as a lightwind board.
What I would strongly recommend you is to take the one or other private lesson with a good trainer. The best 500 bucks I spent for windsurfing in my life was not in the latest carbon boom or another sail, but in 10 hours with a good trainer over the course of an entire season. Best investment for progress and all the fun which comes with it. He will also help you to rig/trim your new equipment which is a bit of its own science...
Welcome in the windsurfing world, you are by far not too old to learn it and have still another 30+ years of windsurfing fun ahead

best regards
mariachi76
(43 yrs, started with 8yrs old but learned seriously only 3 yrs ago, got fully addicted and bought in the last <3 years in total 3 new boards, 7 new sails, 4 masts, two carbon booms and travelling now from Maui over Philipines to Brazil just for surfing

. Be prepared for full addiction!)