You get one noggin'. We care about your brain, and it's a non-negotiable, absolutely essential piece of safety gear. And while there are many useful used pieces of gear, a helmet isn't one of them.
On any given ride you might encounter unrideable terrain. Likely, it won't be for long.
For the rest of the terrain, it varies from smooth gravel, to single track. At least a 40mm tire. Personally, I love a 2.1' or 2.2' tire, if your bike can fit it. It can be helpful to adjust pressure or use a calculator like this one.
Ride with GPS has a free version, and it can be very helpful to take a look beforehand.
Make sure you pack a flat kit, with riders running a variety of set ups, from tubeless to tubed, 26' to 700c, we may or may not have something to get you home.
Minimum:
Pump or CO2
Lever
Tube
Plugs (tubeless)