Another ex-windsurfer here. A inexpensive large board with a fairly large centerboard and a medium sized sail would let you practice in calm conditions whenever you can get out on the water. That will carry you up through winds of 10-15 mph. When you find it doesn't go fast enough for your taste or the sail becomes overpowered, (mast flexes, sail looses shape, etc.) that is the next step in the learning curve and a decision point on finding better gear. My first board lasted 1 summer and it was off into the quest for better gear. I finished up my sailing as my stuff became unreliable in the extreme conditions and it was becoming too much work to load, unload, wash and repair. Typical trip was a full day bringing 4 boards, 3 masts, (with top and bottom extensions) and 7 sails ranging from 3 m^2 to 9m^2 to split between my wife and I. Gear is dependent on the size and weight of the sailor. She was always one board size down and 2 m^2 less sail size for a given wind condition. Enjoyable to just explore a small lake or bay in light winds on a large board or passing hobie cats and jumping waves in high winds on a short board that requires a water start because it doesn't have enough buoyancy to float you, (think large water ski with a sail on it).
I'd recommend starting with a bigger board in light winds and see where it takes you. If you don't like it after a summer, just pass it on down the road.