Workflow across modules

Workflow across modules

I would like to generate an e-mail when a Prospect becomes a client for for the first time.  Our internal finance department needs to set up a new client on our billing system when this happens, and I want to be able to e-mail them all of the information we've gathered about the Account, associated Contacts and the Opportunity while in the business development phase.  The logic would be fairly simple: 

When Opportunity Stage is Win for the first time AND 
Account IS NOT "Client" OR "Former Client"...

Execute the workflow...

But this depends on fields that are attached to both the Opportunity and the Account modules.  Unfortunately, it seems like the Workflow automation only allows you to access fields from one module. Has anyone sorted something like this out?   

We're on the Professional level plan rather than Enterprise.