The expansion is basically needed. Most of the end game content that is different from the campaign is unlocked by the expansion. I have convinced a few of my friends to buy the game and they played for 2-3 months before giving up playing it. I will say it is very addicting starting out but if you have no long term goals it can lose interest after that 2 month period of play.
Community is there but not huge or anything. If you have a friend to play with I would recommend just playing with them as the game is a lot of fun with a small group of friends.
The only thing I would say about playing duo starting out is that there are 2 classes for each primary stat and these classes can share gear fairly easily.
Dexterity - Monk and Demon Hunter
Intelligence - Wizard and Witch Doctor
Strength - Barbarian and Crusader
Beyond that I wouldn't worry too much about which classes you play. If you don't enjoy a class, leveling a new character is easy and quick in this game and your friend can power level you pretty quickly if they aren't interested in leveling another character.