The goal is to get a full Linux environment running on minimal power.
Q: What's a full Linux environment? A: For the purposes of this discussion, we'll say it's a well supported distribution running with a conventional window manager, good enough CPU, RAM, disk, screen, keyboard and mouse to do normal day-to-day task including development.
Q: What is minimal power? A: For the purposes of this discussion it is 5W, that is 1A at 5V. Why? Because that's the sort of power you can get from conventional Power Packs, so enabling long off-grid usage.
Q: What is good enough CPU? A: From playing with the full range of Raspberry Pis, anything fairly recent seems good enough. Specifically, a single core running at 1GHz is good enough.
Q: What is good enough RAM? A: At 512MB things start to swap, 1MB or more is good.
Q: What is good enough disk? A: Enough to install a fully featured environment and have space to play. Let's say that is at least 32GB, preferably 128GB.
It is commonly accepted that:
They are fairly easy to set up, and they sit nicely over a model B. Rasbian has defaults for small screens that are a good starting point. However, the screen size is really small, it's a real strain to read and too small to do any work. The screen update can be slow, perhaps because it's via the GPIO.
The standard 5“ is HDMA, such as this for £23.78. The resolution is 800×480 which is just about acceptable, given all the constraints. I found the power consumption quite high, but I don't have hard numbers yet.
A basic kindle has a 6” eInk screen, potentially big enough. The refresh rate on eInk is very poor (as it the colour) but it does have lower power consumption. However, the oft quoted “30 days” of battery life for a Kindle is a misconception, it's 30 * 30 mins, or 15 hours. The Kindle itself isn't powerful enough (not enough RAM or disk) and it's really not clear that the extra life of eInk is worth the extra processor and comms and display slowness. Here is what it looks like when working http://netbuffalo.doorblog.jp/archives/4609419.html
“apt install xrdp” on the Raspberry Pi and “RD Client” on Android gives a full independent desktop on Android. However, it transmits bit images which are slow over WiFi.
The best app depends on what version of Android is running. For mordern Addroid (currently 9) https://userland.tech/ seems to be best.
Provisional: 5.5“ screen with resolution 540 x 960 (35% more pixels than 480×800). 1GB RAM. £27.55 https://www.ebay.co.uk/itm/2019-5-5-Large-Screen-3G-GSM-Unlocked-Android-5-Core-Mobile-2SIM-Smart-Phone/15341263866