Is it only through the checkout page that this validation needs to occur? If that's the case then I'm unsure if a specific Commerce component is required, in that case I'd say it's simplest to create your own service that checks this requirement based on the information you pass into it through a Cart you're about to turn into a PurchaseOrder. (It will need it's line items set and shipping information set of course).

If it on the other hand needs to be reusable in various parts of your order processing flow, I'd look at creating a new Activity and put it in the relevant workflows. The workflows that I see as applicable are CartValidate (should be called whenever a change to the cart is made) or CartPrepare (should be called just before "accepting" an order).

If you go with the latter approach, the guide that you mentioned should be useful for customizing existing workflows:

( https://world.episerver.com/documentation/developer-guides/commerce/workflows-and-activities/Customizing-order-processing-workflow/ )

To make it simpler, you can download the source code for the workflows through this page.

https://world.episerver.com/download2/

You'll find workflows for your version under the section "Other code samples:" -> "12.3.1 to 12.5.0"

You can use the workflows source code as a reference point to how your custom workflow should look like as a base so that it's easier to add your custom activity to it where applicable.

Hope this helps!