I just ordered an Ender 3 yesterday.  It should be here Tuesday.  

I've imaged and updated a RPi3 with OctoPi.  I'll be going through setup when I get the printer on Tuesday.  

What do I need to know?  Do I need to do anything, software-wise, before getting started?

Do I need to get a RPI 4?  It looks like my 3 should be fine, but others may have experience with octoprint.

I have a 3 and it seems to work just fine - setup was very easy

Same, set up was super smooth.  I have it on a 4, but a Zero will run it just fine also (minus a webcam)

whole bunch, we run them with Octofarm.  Fire and forget prints, check in with cameras, all networked nicely in a centralized system.

Don’t forget to assign it a static IP address on your router so Cura doesn’t go looking for 192.168.0.123 when this week your RPi is suddenly at 192.168.0.456

I see what you did there.  

Static IP set.  I set it for all printers and servers.  I consider this both.

I should do this for my Ender3v2.  Does it work with the onboard screen/computer, or do I need to get a keyboard, mouse, monitor and all that jazz for the Pi?

(Fun Fact: My very first print job years ago was to make a case for my 1st gen Pi.)

It's accessed via web interface.  My octopi is headless.

Add already mentioned, octopi is access via web interface...  However, if you compile your own Marlin, check out this TeachingTech video on how to control the PI (shutdown) from the printers LCD, along with a few other tricks.

me i got mine on a pi 4 2gb and runs fine on my ender 5 when i need to run it. there are some plug ins that are nice like better print time calculator and some other things but i pretty much run it stockish.

mine has the webcam and is static over wifi so really other then power and usb to the printer are needed. even have the webcam set up to connect to my security camera system to ether watch threw that or record. one thing i did find out is that with my web cam [Logitech 920] likes to variable focus so have to command line in to set a set focus. https://community.octoprint.org/t/anyway-to-disable-the-autofocus-for-cameras/801/11