My zone is different, so my experiences might not help.
Peonies of course, need full sun. Most all hydrangea require more water & shadier conditions, at least in my zone they do. Salt-I can't think of any plants that can handle much salt really, but there may be some.
Since you can't wait til fall, I would dig as soon as possible. I doubt if yours are in bloom, but if they are that far, then you might as well wait til after they bloom & enjoy them. I know everyone says to divide in fall, but I divided mine once right after they finished blooming & divided them. They didn't look good the rest of that year, but they did bloom the following spring. That was before I had a clue how to divide peonies.
Ants shouldn't be a problem. I have found them on mine, I think they are just attracted to the sweet pollen, but just natural.
I think clematis & phlox would be quite pretty, you should have a lovely new garden-if the salt doesn't prove a problem.

The only hydrangea that can take full sun are the paniculatas. Most people are referring to the macrophyllas when they talk about hydrangeas--and macrophyllas must have part shade. The paniculatas are gorgeous if you want to go that route, but they are much too large for the space you describe--at least all the ones I know are. They bloom later in the summer than the shade-loving macrophyllas do.

Peonies certainly don't like to be in soggy wet conditions, but I'm not sure they especially "like" dry conditions. They are tough, long-lived plants that tough it through a number of "difficult" growing conditions, but like any plant, they probably thrive and bloom better if they are treated better--which would include occasional moisture.

You are aware that most peonies grow 2.5-3 ft fall and wide (or thereabouts), aren't you? I ask because you mentioned planting them in front of a small fence. I was having a bit of trouble visualizing that. Just warning you in case . . . : )

Put a couple mums in there for autumn bloom. That's what I do, anyway.

Good luck with your plans--half the fun of gardening.

Kate

Yes, the height of the peonies are just what I'm looking for. I'm thinking about putting in a rail-type fence and want a hedge in front of it. I'm really leaning toward the peony now but would have to see how they do up against the salt. The hydrangea folks gave me a couple of suggestions for paniculatas to look up, too. I have some small peony already and they are just starting to poke up. I don't think they bloom until mid to late may if memory serves me correctly. It looks like I'll have enough plants to make a hedge right now.

I will water but I don't think these will look as droopy as a hydrangea if I get a tad behind.

I just read a little about powdery mildew - it's a fungus. It seemed to me it was caused by heavy rain but apparently the fungus thrives more in moisture so it was a combination of the two. There were a couple of good recipes with baking soda and water or baking soda with horticulture oil. Does the fungus die over the winter with the foliage or is it certain to come back with new growth? I wouldn't want nasty grey plants in the front yard. I could start spraying them now, as they are growing if I need to. It was recommended to spray every 10-14 days. Is that something that would have to be done all season??

I found my answer here to the powdery mildew when I searched. It said you have to spray for 3 10-day cycles. And, as long as my soil didn't get contaminated, the fungus went away with the dead foliage. I just checked them and didn't see any signs of mildew. It been unseasonably warm this winter and I see lots of buds swelling already. Better wait if I plan on moving them so I don't miss the flowers!

one annual you may like is caledula.. tough & self-seeding, so you only need to plant them once. They bloom all summer & often into fall.

Somewhat short, but tough, are the sea thrifts.. as their name suggests they have a good salt tolerance.

Chrysanthemum are photoperiodic, so if they are too close to a streetlight they won't bloom.

My peonies line the front of my yard, perhaps 8-10 feet from the street which is kept very well by the township. They've endured salt for years and just keep getting bigger. I think as long as you plant far enough back for pedestrian traffic you should be okay.
I grow Walker's Low catmint in front of the peonies, it gets cut back after the first flush but otherwise seems to bloom nonstop. It might not be the most dazzling plant but it does seem to pair particularly well with peonies...

Jeanne

Great suggestions, everybody! That's a very interesting tidbit about Chrysanthemum, freki. It looks like your peonies are doing awesome, Jeanne! Thanks for all the responses. I'm going to give these a try. I have some catmint; I like that pairing. It does have a long bloom time. It self seeds like crazy for me, too if I don't give it a haircut.

Kittens

Re the powdery mildew I use sulpher on any plants with powdery mildew or black spots. I use it diluted in water on the actual plant. Soak the upper foliage and as much of the undersides as possible. If it rains shortly thereafter repeat the process. You can actually see the mildew drip away. In the fall I clear all dead foliage and sprinkle the sulpher powder directly on the ground as the powdery mildew spores can overwinter. If I forget to treat the ground in fall I do this in early spring. My summer phlox which are very prone to mildew always look great all summer. In a pinch if I'm out of sulpher I use baking soda diluted in water. Never use the baking soda in powder form as you will kill your plants. i speak from experience.

Jeanne in Ohio...please tell me you know the name/variety of the two peonies you showed in your picture. The last picture looks like the peony that the previous owners left me and I love that first one. Is it a Sarah Bernhardt?

How deep is your flowerbed where the Walker's Low and your peonies are planted? How close is that Walker's Low to the peonies and how much sun do they get? That blue against that pink sure is certainly a nice combination. Are either of these plants deer resistant?

I sprinkle some Zinnia seeds in May. They grow up into the bush and it looks like the peony is blooming beautiful different colors.

I purchase Burpee Giant Zinnia.