Computer systems:
Pi 3 for enough power to run your normal stuff … 5v 2a needed to power it.
Pi zero is a bit smaller, but still powerful, and you need to plug in a wireless card ($20) to connect to internet

OS and data:
Everything goes on a microSD. OS (called “Noobs” Raspbian Linux) is about 2gig I think. You can also put your OS on a flashdrive or connect to an external data source otherwise I think.

Monitors:
Pi official screen (about $80 or $100) – 7inch tablet size touch screen … 5v 2a needed to power it (in addition to the computer, but I have powered both (they link together in various easy ways) with a single 5v 2a power source (wall/battery), although I do get the lightning-bolt indicator flashing the whole time (signaling the computer needs more power)

Power:
5v 2a batteries / backup USB power sources (the ones usually sold as backup power for phones), some with solar power options. Various mAh (milli-amp hours, or how long the battery will last drawing the amount of power it’s rated for), from 3000 to 25000. Some have remaining juice indicators on them.

papirus-hat-9-1000
A low-power (almost zero) small screen but it doesn’t refresh at a decent speed and has limited resolution, meant as a notifications screen, from PaPiRus.
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A 7.5 inch eInk one.
For big (15, 25 inch) screens, there are USB monitors that do low power draw – some have low-ish resolution though, and all are expensive as of 2018 (hundreds of dollars)