What about the Healing Wand?
Can I also purchase the AA Skill first?

Cruel.

Yes the order of buying AA or items doesn't matter.

I don't know why it took me so long to write this, but here you go:
EH only works as advertised (EH lvl * Op. lvl * 0.2 * (1+ CA*2/100)) if you are not attacking a much lower level player. Even though I believe this is fine given it is a very reasonable balancing measure, the skill description should be fixed to describe how it actually works. Also, it wouldn't hurt to disclose the correct formula for when attacking lower level players.

Thanks,
Cruel.

I believe the formula is OK up to level-3.
Attacking level-4 yields 80% on your formula's SE, level-5 60%, etc ...