I think in general, the old cheap oldies are a pretty vigorous lot, at least the ones that are still around. They're still around for a reason, they've been time tested.
There's also the water and soil issue. My version of zone 6a/b living near Boston, is not your version of 6a/b. We get a lot of moisture here, usually, all year long. I think the annual average rainfall here is 47 inches. We also get another average of 48 inches of snow too. Daylilies respond differently to that than to dry conditions.
I agree that the best thing to do would be to find clubs or growers in Arizona. That's where other folks who have been at it for a while can point you at successful plants. I do believe you can find Daylilies for every climate, you just have to hunt around.
