Iron Brigade's Rise of the Martian Bear is a must-own if you have Iron Brigade. I honestly wouldn't recommend buying Iron Brigade at all if you're not getting the DLC too. Martian Bear adds an epilogue story, but more importantly, 10 more experience levels to progress through, several new campaign levels, and lots of new equipment. Oh, and new Achievements.
Iron Brigade actually does have a loyal community and it shouldn't take more than 5 or 10 minutes to find a group when playing during peak hours. You have to remember that games go on for longer than in many titles, so a little patience helps. Anyway, if you're stuck at level 10 or below and can't play the new missions, you'll have a harder time finding a group. Go all in and you'll be happily unrestricted.
Also, the Martian missions aren't that tough as long as your group is decent. A little building up in survival is actually important in this game, and not some optional thing you should ignore. Survival gives you the EXP and credits you need, plus exclusive equipment, some of which is fantastic. Anyway, build up properly and your team won't have much trouble with Mars at all.
Did I mention that Iron Brigade is a game you play cooperatively with friends/other players, and not very much by yourself? Yeah.