May seem like a good idea but (personally) DO NOT buy it. Given that you just came back to the game, there won't be enough time to finish ROI. Just wait until Destiny 2 comes out over the summer or fall and but that. It would be a waste of money to buy it, now that the sequel is on the horizon.

If u want the full experience buy the content

I would suggest buying it - the PVP is fun, as long as you play objective and wreck people with your awesome skills, you will have fun, especially if you are not playing for gear or weapons - there is a ton of unique ways to stay creatively engaged in PVP. I do want to mention that if you completed the felwinter's peak quest line (I think that's what it is called), Lord Shaxx in the tower will give you a weekly objective bounty to go out and complete - I find some of them to be skating easy, others are a challenge. It is worth it.