Goggles - a bit obvious, but comfort is the most important thing,closely followed by decent anti fog properties. But any new set of goggles has a decent lens, and can most likely be upgraded with 'better' lenses. Only a cheap set needs an upgrade lens, but even a cheap set could be comfortable
With second hand be prepared to buy a new lens, so factor that in to make sure second hand doesn't become expensive
Comfy clothing - you should have some already, no need for paintball specific. But depending on how you play (eg sliding around) then you may benefit from specific clothing
Gun, air system, hopper (unless you're into mag fed)
Big bag, ideally with wheels
But you can make do with a backpack
Pod belt or vest and pouches
Pods
(Again, not if into mag fed - in which case you want magazine pouches)
Then things like 2 microfibres (one wet to clean, one dry to finish off),fluffy barrel swab,barrel condom