Hi
When we do flower arches we generally create a evergreen garland, which is foliage bunches wired around a rope which we then attach the flowers to when it is situ.
You would then hang this from the hooks you have. If they are in the right place the weight of the garland will hang it straight which then could be fixed / weighted to the ground to stop it moving.
Churches officials are, quite rightly, very sensitive to any structure put on their old churches, maybe a word with the flower ladies might help as undoubtedly someone will have done this before.
As for price, that would depend on the flowers you were putting into it, we charge between 12.50 and 17.50 per foot for ours. Plus a set up charge.
Hope this helps
Kay