1. You can pretty much play with anyone that's a higher level than you. The game scales the higher level player down pretty well.
2. Yes, some of the stuff will be different. For example what you select as your back story will come up in the storyline as a few quests. Also when you select an order that bit will be different, but the overall story is similar for everyone.
3. I've had a good time at EU server Ring of Fire. I haven't played in a while, mostly because I've gotten distracted, but the community there is great in my experience.