The quest you have to do, Priest In Peril, will only get you to 11 pray.
Now if you want a pure to have a good outcome, prayer always helps.
Plus, to do Desert Treasure(DT), it would be rather difficult without Prayer, it would be difficult to do 1 or 2 of the required quests without prayer too.
So I'd suggest getting it, getting it high as well.
Either way you're going to get prayer, and would you rather jump a few combat levels now and still have a chance at owning or would you rather wait until you've got 80-90 str and you then jump a bunch of combat levels then?
It all depends on when you want to do it, but that also affects how soon you get your ghostly robes.