You can put them out on warm days and bring them in on cold days and nights if you want. They will do better that way and take less hardening off since they have broken dormancy than if you try to grow them by a basement window. A wheelbarrow to easily move them into and out of the garage, or just take the plants out as you head out to work if the temperatures are above freezing. If it's any comfort, after my grandmother died, I divided and brought back a couple Festiva Maxima peony roots from plants that my grandfather had planted many years prior. This was mid-July, and they survived and grew just fine after a couple of days wrapped in damp paper towels. So I imagine that at least some of yours should survive.

Unless it had three eyes or more do not expect any peony to "do well" next spring. It will take at least another year before it will start looking like most of the peonies you see in other gardens. There is nothing for anyone to do with this peony other than let it do what it wants to do. Some would suggest removing the spouts but the plant will do what it wants to. If you remove the sprouts it may force the root to form new buds rather than sending out feeder roots. FYI Most peonies bloom buds are formed in a three year cycle. The buds that will bloom were formed three years ago. It takes that long from erupting from the root to becoming mature enough to form a flower bud.

If you purchase a Sarah with buds on it and want to see blooms this year you'll probably want to leave it in the pot until after the blooms are done. You can get away with putting a potted one in more shade than you'd plant it out in--actually preferable since it's probably been greenhouse grown and could use the hardening off. Make sure the pot is shaded so the roots don't overheat, don't overwater and it'll be good. Once it's done flowering you can plant it any time you like, just like any other potted perennial it'll need some babying until it's established. Check to see how deep the top of the roots are before you put it in the ground, you'll want it to be around 2" deep in your zone 5b, no deeper. That's where the eyes will form later in the fall, on the crown of the root, and if they're too deep it can inhibit blooming. Everyone has their preferences with planting, either bareroot or potted. I have 2 sources that I'll plant bareroot in the spring but mostly prefer fall from growers who dig/divide/ship in a fairly tight period. Peony roots can survive a ton of abuse though, saw a post on the Southern Peony blog where she found a box from a few years before that she misplaced with roots in it--planted them and they grew!

You can dig up the peony and transfer it to a new spot even now - if the ground is not frozen solid. Also do not plant the bare root peony in a planter and keep it in the garage. As long as you have a viable " eye" on the barefoot, you are better off planting it in the ground. Peonies need to get the winter chill to flower. So if are still undecided on the final planting site, put it in ground in the vegetable garden. Then transplant it in the spring to the planting site of your choice.