The Atmel 328P - xx has an absolute max operation specification of -55 to +125 °C and a storage temperature of -60 to +150°C (i'm looking at this file: Atmel-8271-8-bit-AVR-Microcontroller-ATmega48A-48PA-88A-88PA-168A-168PA-328-328P_datasheet.pdf)
Atmel appears to not make a military version of this device (maybe not any device).
It also states that with a 4 Mhz clock the chip will operate down to 1.8V
I personally have no issue with pushing the limits on a device for a one off project and I've seldom had any issues. However one also must consider the consequences of a failure. For me the worse the consequences the more conservative I become. I'm sure you would be bummed if your data logger failed after one or two days.
Below are some thoughts that might help.
- The most likely failures in cold are mechanical (bad connection) and a failure to start coming out of sleep.
- When purchasing a board the thing you want the most is good workmanship (soldering). I've seen many boards with simply horrible solder connections. Fine for a hobby but won't stand up to mechanical stress.
- Thermal cycling is cumulative if your devices passes one cycle it does not guarantee it will pass 10 or 20 cycles. Also, the speed of the temperature change is important, the faster the temperature change the greater the stress.
- If you can eliminate the DC-DC you would be much better off.
- For potting you would be much better off with an electronics spray silicone conformal coating. If that is not an option folks have had good luck with GE II. Make sure it is absolutely positively cured if you wish to then pot it in epoxy. As others have mentioned you don't want to pot the board directly with a hard potting.