I am 50, but with decades of board experience. The full face helmet is really overkill unless you plan on testing the speed limits of the board. If you already bought it, you might as well wear it until you gain some confidence on the thing.
In order of priority I would say helmet is essential. Don’t even balance in your basement without one. The hardest falls I’ve seen localy haven’t been nosedives. They have been dismounts gone bad. Wrist guards would be next highest priority. Not so much for protecting against fracture, but more to keep from having to pick sand and gravel out of your palms if you take a header.
Elbows are next and then knees if you please.
As far as skids and skins, save your cash. It’s a onwwheel, not your grandmas bone China. They look cool with some “experience-marks.” Plus the skids take away some clearance on the XR. Less is more.
No balance board or exercise is going to prepare you for the riding experience, like riding it. You won’t test the battery limits on your board for quite awhile. Ride it on multiple short rides (15 minutes or less) over several days, to build onewheel endurance. If you take it on an hour ride on day 2, it will sit in its stand on days 3-5. Also, the fatigue that initially comes with the riding stance leads to newbie safety issues on longer rides due to soreness and initial lack of stamina.