It has PvP but it is largely focused on PvE, PvP is not that big. PvP also has it's own set of rules and gear works differen than in PvE. I don't think many players play PvP. No, you can play just fine without ever paying a cent. Just playing the game will probably be enough to get upgrades, gear and new frames. It has some story but it is not a big deal. Having a clan is a must, it gets you acces to some gear not obtainable otherwise. Crafting is a big part of the game. And I like it because it is a nice side game, pick up and play for an hour and then do something else, but you can still binge it once in a while.