How to limit access to certain nodes to certain users only?
Depending on how much more deep you want to take the logic of your project, you might not even need custom coding.


Start with Drupal 7 minimal installation profile
Install the modules views, entity, entity reference and their dependencies.
Create the content type "order" and add the fields you need to it. Add other content types as well like "invoice" and document, these last you might want to add a "reference" field to the order content type if they are connected.
Setup the user roles "client" and "supplier" and you might want to install the "ACL" and "content access" modules as well to define who has access to what, like "only"3 the author of the node can see it, edit it and delete it.
To make it easier for the user to see the nodes they have access to, you can create a tab on their user account using views listing only the node where they have access or where they are the authors.


Notes:
You might want to test the system before going live to make sure the right person can access the right content. Also if no anonymous user is meant to use the system disable content access to the anonymous users.