I'd go with a Pi 3, 7 inch touchscreen, and 3 - 10 18650 lithium cells. You can get little switch mode voltage regulators on Aliexpress or eBay for a couple bucks. When you're not using it as a portable you've got 4 USB jacks, WiFi, Bluetooth, CAT5 ethernet., HDMI. Not sure about Debian drivers for the display, they come with Raspbian (almost the same).
I've been thinking about what a handheld computer COULD be.
My image is heavily influenced by my recollection of Palm Pilot.
My project goals are two-fold
1. create a personal data logger reminiscent of a Palm Pilot
2. become familiar with Raspberry Pi while using Debian as the OS
My needs include:
>2 hours battery life
4" by 7" nominal form factor
touch screen input using a stylus
display will be entirely character mode (40 chars/line would be OK)
OS GUI not required except to say where stylus is
OS shall be Debian {possibly with non-free drivers}
I've not found found user friendly selection guides.
A typical problem was not being able to know if a selection of components had mutually compatible I/O (electrical and physical).
Suggestions for suitable selection guides?
TIA