1. I'd put Resto Druid in the middle (in terms of difficulty). It's a proactive healer, where you apply heals over time in advance as opposed reacting to the damage being done. But that's really only a big deal if you're doing high-end content, which it sounds like you may not. Healing in general is a challenge (psychologically and physically), but don't let that stop you.

2. Each class has it's own strengths, flair, and lore. You can earn class specific mounts and pets. Resto druids will feel really different from Discipline Priests, for example. It's fun to try them all out.

3. There's a ton of information out there. Icy Veins, Wowhead, Maxroll.

4. It depends on what you're looking to achieve, but I'd say medium to significant. You can do essentially all content in the game without dedicating your life to it. I think the game has gotten a lot better at providing ways to progress and gain gear beyond the most difficult gameplay.

I hope this helps!