Workflow that checks for or requires specific documents

Good day all,

Wondering if anyone has a way to check that certain documents exist before a workflow can progress to the next state? For example, I want to check that two or more documents of different classes have been added into the M-Files vault (with a common piece of metadata such as "Order Number") before the user can move a workflow into the next state.

Long story short, we're trying to enforce a workflow that requires certain documents (sales agreements, service agreements, etc) to be present before our sales people can submit the order to our order desk.

Has anyone done this before? Or is there a solution or add-on that handles these kinds of rules?

Thanks!