I'd start with three shaman, elemental spec.
Down the road, when you add two more accounts as the finances allow, the 3 shaman can be the core of your first team.
You have the option of a tank + dedicated healer.
Or of a tank + boomkin/warlock (for the 13% spell power boost) and use the shammies for DPS and Heals.

A shaman can boost through Scarlet Monastery very easily.
And probably Scholomance/Stratholme, and then Ramparts, if the toon is 80th with a little badge gear.

Once you reach 60th, you can use totems to boost to 70th fairly easily too.



Three of the same will be the easiest.
After the shammies, consider what other teams you'd eventually like.

I'd probably level 3x DPS as a group of three.
And down the road, add two more accounts and RAF boost those to join your existing set of 3's.