I have an aoe grp. Prot pally, 3 arcane/fire mages and Disc priest. It works great. Dunno about the others cos im horde but SM was sweet. I stayed there from 28/29 till 41 and never got bored once. It is def easier and quicker if you are aoeing mobs a couple of levels below you least thats what I found.
I would def go priest for your healer so you have holy nova. It totally kicks ass! I consider it absolutely vital for what I do.
I would also def go 3 mages. Ive aoed all instances from RFD to BRD with 2 mages and lock and 3 mages is way better imo. Plus it is very handy being able to move and aoe. I wouldn't worry too much about repairing i hardly ever did and it was still dead easy.
Can't remember how long it took me to 40 but i'm almost 60 and sitting at almost 5 and a half days. I've always thought I'm quite slow at levelling as well.