How to power my datalogger?
If you are using a microcontroller then you can implement some of what you're looking for just in firmware. Namely, the automatic low voltage cut-out is known as brown-out and is something you can set fuses for on the AVR chip itself in which a RESET event will be triggered. However, you might look into a latching circuit that could be switched off when a RESET occurs. See page 47 in the datasheet.

The blinking LED is about the simplest program you can write. Once low-voltage is detected, either through brown-out or ADC, drive an LED with PWM and a timer if you want it to blink. If you simply want an indicator then just drive the LED high and forget about it.