I think 5x Priests, will get you consistent points each week.
You'll raise up the rating ladder a point.

But I'd be very surprised if the team were to get 1800.

It won't be a competitive team, in arena.
But can use arena for faster gear.
And won't be horrible in battlegrounds, especially in a support role.

Mass Dispel with Fear Ward on a round-robin, is some fear bomb protection.
Not to mention they can AoE fear too.

Every team/composition brings strengths and has weaknesses.
You need to identify the strengths, and think of ways to use them.
And also counters to your weaknesses.
There are a lot of compositions which can work to a degree.
And it also depends on if you'll be focusing on 2v2, 3v3, 5v5 (arena matches) or on BG's which will be rated in Cataclysm.