To get level 40 will require missions and bounties etc, similar to getting to level 20. So you will be able to access all content in TTK as far as I know. So if you enjoy the game, I would recommend buying it. As for past and current content, almost all of it can actually be finished solo. So it would be fairly easy for a group to help you complete anything ( with the right group ). If you want to just see them, I suggest at least going through the normal versions. Even PvP is not unobtainable. Iron banner can be farmed and you can eventually buy some etheric light. There's quite a few people who will also help people go through Trials of Osiris which could help you get some gear and/or etheric light. As long as your group knows in advance they should be able to plan for and overcome almost any obstacle.