I have a recollection of reading something about overwintering plants in containers saying to consider your plant as growing 2 zones colder than what you're currently in. If that holds true, then being in zone 7 with roses hardy to zone 5, they may be just fine. Regardless I think with minimal protection you'd have great chances of keeping them.

Mika, Austins are a bit pricey here too, and almost always more than the rest of a vendor's offerings from other breeders. The bare root plants from the Austin site were around $25 each (about $5-$8 more than other bare root roses from other vendors), but potted plants at garden centers are closer to $50.

Hmmm...I'm kind of lost here. My nice expensive clay and ceramic containers cracked and broke after freezing one winter, so I've been converting to plastic containers since then. I found some foam types that look like concrete. A few of the nice ceramic ones just had cracks, so I put a plant in a basic black plastic pot, then set that down inside the pretty container.
Are you thinking the plastic pots won't make it through the winter? Or that they don't provide enough insulation?
For the heavy concrete pots I use for annuals, I cover them in the winter so they don't freeze, expand and crack. On most I just use a big plastic saucer and set a brick on that. In the pots that have roses, if there's space in between the black plastic pot and the good pot, it does fine all winter. Just as Neal said, if a rose is rated to zone 5, it does ok in a pot in the winter. Drifts in particular grow well potted. I love how they "drift" over the edge.
Right now, Lowes has some huge colorful (jelly bean colors) plastic pots for $10. REally huge, big enough to grow a tree for years. They also had some plastic whiskey barrel looking ones. They have knock outs for drain holes. I'm using one to grow a small lotus, and another with the holes knocked out to grow a few new roses.

The price of the bare root Austins here is £16.50, so just about the same; potted ones are cheaper here, at £21.50 - £23.50 ($31 -34). All grafted. Standards are £40+ ($59) bare root, more if potted. All these are David Austin prices - many of the older varieties are available more cheaply from other suppliers. So not cheap, although some of my plants are almost 20 years old - on that basis, pretty good value!