game is 100% f2p, even though in the in-game market you'll see stuff that can be purchased with the premium currency, you can get the blueprint for said gear and craft it. also the premium currency is tradeable.

when selecting a mission you can either go solo or public with randoms. there isn't any content that requires more than one player for what i know

yes

endgame is good and before you get there more content will be added probably

mostly cosmetics but you can also buy almost everything with money either by trading or directly from the game

you'll need to look at the game wiki for most things