The co-OP is great. There are power level requirements to go up in difficulty but nothing to stop you going back down. If you have a friend get it and you're much stronger than him you can play with him until he catches up no problems.
No endless mode, there is a loose story but the main game has 13 missions with 2 in the boegenhafenn dlc 3 in the back to ubershreik plus an unlockable horde mode so to speak. Winds of magic gives one new map and weaves which are kinda meh but they're there for more content. Ubershreik and Winds also give you 5 new weapons apiece. The grind isnt terrible, getting all five heroes to level 35 can be a chore but it's not super terrible.
Best tank would be Ironbreaker Bardin. Support for me is Mercenary Kruber. Dps is harder there are a couple of classes that do it better than others like Zealot Saltzpyre. No class is bad, most weapons are all viable some just take more work to make them viable.
Have fun.
I just wanted to note that we recently released 3 new levels for free that takes place in and around castle drachenfels