How do I decide which ARM board to go for if I am aiming to use it as a micro-controller to run sensors, actuators, motors, and solar panels? And I wish to power up the board via Li-ion battery pack.
I guess you can connect sensors, actuators etc. to any ARM board as long as you have proper interface circuits. So you will need external driver for motors, signal processing for the solar panels and so on. I am not yet aware of a board with these circuits on-board.
You will need to find out the number of I/O required and any other feature you need such as LCD or Ethernet or USB etc.
As for the low battery, pretty much all ARM processors are quite good with low power requirements.
Have a read at the blogs by carissal - A number of boards are reviewed and explained there.