That seems a sensible way to go.
Rather than try to put emonCMS onto the SD card you bought, I suggest you flash (“Etcher” is the tool I recommend - there are others) the SD card with the downloaded image, which as I mentioned, is a complete working system. We will all know your starting point if you do that. Etcher is very straightforward - the SD card must be 16 GB¹ or bigger, and if it’s bigger, then while it’s in your laptop, use GParted to expand the data partition (it’s the one that is not boot or rootfs) as far as it will go. GParted is also fairly obvious but make absolutely certain you are looking at and working on the SD Card - or you’ll screw up you laptop, possibly fatally. The only catch with GParted is you pick what you want to do, then start it off as a separate operation.
Also, while the SD Card is in your laptop, you must enable ssh - see emonPi SSH disabled by default
or, if you want the really hard way now but a lot easier in the future, set up a pair of “key files” on your laptop & the RPi: Command Line Cheatsheet - #3 by Robert.Wall
You won’t need a screen and keyboard connected to the RPi (I’ve never done it that way. 
)
I’d also modify step 2, to
Put the Wi-Fi credentials in while it’s still connected by Ethernet. Setup → WiFi. Drop the Ethernet connection and check the Wi-Fi.
Then, connect at least one of your Modbus meters on the bench and get it working with something appearing, first in emonHub and then on the Inputs page of emonCMS. (Note: you can’t create an input in emonCMS, they appear when valid data arrives.)
Only then would I think about installing it in it’s permanent home.
There’s a lot of fairly recent material about RS485 connections and whether an earth is needed
The Guide sections, that are nearest to what you need, look to be
https://guide.openenergymonitor.org/setup/connect/
and
https://guide.openenergymonitor.org/setup/local/
¹Note: If your SD card is smaller than 16 GB, it can be done (I have an 8 GB image) but it is tortuous to shrink an image, whereas it is trivial to expand a partition once it is on the SD card.