but I don't think I'm even done with all the story missions. There's a nice chunk of content.

This being enough to bring you back to the game probably depends on what made you take time away from it, and when exactly you stopped.

If you "beat" the game, got the rifle from that robot and stopped, you'll probably get a lot out of buying the version that comes with everything.

I'd say maybe buy that disc and then if you don't like it you can always trade it in.