When is it too cold to risk buying fertile eggs to hatch?

We're hatching a new round of chicks this year, after our banties from last year keeled over in the first bitter cold snap we had this fall. :'/ Last year I was so new at it, I didn't think anything about time of year or temperatures outside, so I got eggs New Years' Day and actually hatched 18 out of 24. This year, we're buying from a local breeder, and I was hoping to get eggs next week to start incubating. But our high over the next ten days is supposed to be 43*...ten days from now. The high over the next week is hovering in the low to mid 20s.

So my question is, for the best shot at fertility, should I give it a couple of extra weeks? I don't know this breeder personally, so I don't know how often she collects (more than once a day, I mean) or what her set up is for her hens, if the eggs would stay relatively warm after laying, etc. I know she's got 90 or so hens, and 8 roos. I'm currently planning to buy 3 dozen eggs in the hopes of yielding some sort of laying flock at the end, along with a given set of birds for the freezer. But I'd really like to give it my best shot for a good hatch, so I don't want to jinx things by getting the eggs too early when it's so cold. Or will they be ok, and the temperature shift from this week to two weeks from now won't be enough to make a huge difference?

Any input? Thanks!