The LattePanda has an arduino built right into the board. No need to use another micro controller for this task.
You can program it to act as a usb keyboard and mouse, even going as far as to making it use Win+D and Alt+f4 to and some other key combos to shut down the PC as to protect the battery from overdraining it. Or have it have a blinking light or both.
You can still use another microcontroller for other stuff.
And sorry about my wording.
I'd use a 2+ cell lipo battery, 1cell is 3.7v, 2cell is 7.4v, and 3cell is 11.1volt
I'd use a step down regulator, much like yours, but i'd use one with a low power cut-off. So it doesn't drain your li-po past the safe point. Generally you don't want to take a li-ion/polymer cell below 3volts per cell usually.
So you'd use a step down regulator with a configurable li-po safety cut-off. Or you could make your own shut down code using the onboard arduino, that makes the LP shutdown automatically when the battery is too low.
7.4v 2000mAh lipo battery is still pretty small. With other stuff, you might get an hour at most.
I'd personally use a 7.4v 8000mAh.
Though i'll personally be using an 11.1v 12Ah - 16Ah battery, but i need mine to last many hours.
If the panda takes 2amps, and your other stuff takes 500mA, you can do the math. so a 7.4v 2Ah battery may be too small.
7.4v @ 2000mAh = 5v @ 2812mAh with a 95% efficient stepping regulator. with 2.5A power consumption, that's 1.12 hours.
Lead acid: 6v 4Ah -> 5v 4.5A with a 95% effecient regultor. That's 1.8 hours. This battery is much larger and heavier though.
3 cell lipo,11.1v 12,000mAh (probably still weighs less than your 6v lead acid battery.)
11.1v @ 12Ah = 5v 25,308mAh @95% effecient regulator. = 10.1 hours at 2.5A power draw..
the math goes like this.
7.4 ÷ 5 = 1.48 (7.4v ÷5v)
1.48 * 2000 = 2960 (2000mAh -> 2960 mAh)
2960* 0.95 = 2812 (voltage regulators aren't 100% efficient, switching regulators are usually 85%-95%)
2812 ÷ 2500 = 1.12 ( 2812mAh battery, 2500mA power consumption = 1.12 hours.)
Your power consumption will vary from other stuff you have connected. I'll be pulling closer to 3amps, so i'll end up using a 95% efficient BEC or something that can handle 5A. and some BECs have built in sensors for Lipo batteries.
Long post i know, i try to make sure people have all the information i need. I'm much better at electronics than i am at programming etc.