i got a strange answer for you, when you can't decide which to play, and you "think" you like to do all three things...

go Paladin, shaman, and Druids

all three can do all what you like, healing, casting almost like a mage(but not in big numbers), and melee, because they are the hybird class.
they can do it all, BUT can't do it very well .

the sad part of being druids, you can't see your Epic weapon/armor, since you spend most of your time in other than human form .

the sad part of being a priest is when pug(put up group/group with someone you don't know) in an instance, some jerk dies, he'll just blame you when its obviously his fault. although this RARELY happens, but its just ruins the moment.

i play warlock because i DON"T want to play to the "good" class anymore, and once i got the taste of "evil", i never look back .

the BEST answer for you:
is to ask your friend to explain how every class works(not in wow, in RL or by phone) since you help em' get the mount. its only fair that she help you understand how each class works. she IS your friend, not using you to get the mount... right ?

playing the class that feels right is important and a very enjoying experiance. and can last for a life time

P.S. you friend playing wow is a "she"? cool.