1)Annabelle probably will be the most economical solution because a) it is stoloniferous and will increse and increase in size every year and b) in 7b you could propagate it twice a year, in spring and in a late summer.
2)If you want a more or less formal hedge use 'all the same' cultivar whatever it will be.
3)HD or Lowe will be you first choice of supplier due to economical reason- from them you'll have much larger plants for the same money than from on-line or catalogue retailers. Plus you don't have to pay for shipping and will have a great return policy.
Good luck.
P.S. My personal preference for tidy hedge would be Preziosa which is fairly common in our local HD. If your soil is acidic it will be very nice purpl(ish)-burgundy or burgund(ish)-purple.

Since you are concerned with the WOW Factor, do not forget that hydrangeas are not evergreen and therefore the WOW Factor will be "dormant" for a little time during winter. If this is a problem, you may want to use evergreen shrubs.

No question that I would use Limelight. See photo at the link below. You would have the lovely creamy white flowers in the beginning and end with the magnificent cream, pink and burgundy colors as the bloom matures. As mentioned, be aware that you will have no leaves during the winter. If that is a problem for you, then definitely use Loropetalum or Nandina (not the bamboo kind). Both of these keep their leaves during winter and do well in zone 7. They both have fantastic fall and winter colors. Nandina doesn't bloom but Loropetalum does have fringe blooms all along the branches (gorgeous).

Here is a link that might be useful: Limelight Hedge

I vote for Limelight too. I'd also suggest Pinky Winky! I'd like to have some of those two together. I think they'd complement each other very well.

what about a limelight hedge in the back fronted by a mixed mac. hedge--many of the that you prop. from your exsisting macs????

Yes, I realize that they aren't evergreen. I really meant "Wow" when they are in bloom. I really LOVE hydrangeas, so want to be sure to invest in a good variety.

I like the idea of a double hedge. Is limelight tall enough to do that with? That definitely would be beautiful.

Thanks,
Joy