Here's a question - how small of a group could you get away with later on? 2? 3? 6?
D&D has been aimed at the small group (4 people) section, as that covers all the bases (Pally/Fighter, Rogue, Wizard/Sorc, Cleric). Will you need much more than that in order to keep going at higher levels? Yeah, I realize you won't be able to tackle some of the end-game 'raid' stuff (unless it starts to scale with how many you have in your group), but how far can you go with a small group?
This is just like D&D in that respect. The missions are aimed at a party of 4, though you can have as many as six people. So when a mission is rated CR3, they mean it's balanced for four level 3 characters.
As far as getting a good group, I rarely had to wait more than 5 minutes. All I did was type "Adults please." into the comments session of the group matchmaker UI, push "Looking for a party" and I usually couldn't even make it to the shopping tent before I was in a (usually good) group. This is with a multitude of character classes, not just the stereotypically 'desirable' ones. Also, I really came away with the impression that no oarticular class is needed. Clerics didn't seem overpowered to me at all.