Hi RBL,
Appreciate the update so that the issue becomes clearer.
Office 365, the cloud based service, provides users with Exchange Online, Lync Online and SharePoint Online. The license of Office Pro Plus is included in some Office 365 plans. But for Office 365 Small Business plan, there's no Office Pro Plus license. If users does not have any Office applications, and they want to use Office with Office 365, then users can purchase the Office 365 Pro Plus subscription to get the Office Pro Plus.
Based on your situation, if you want to use Office 365 Small Business, then there's no need to purchase additional Office 365 Pro Plus licenses because you alreay have Office 2013 installed. Furthermore, if you want to use SkyDrive Pro sync client to sync data with your SharePoint Online, then you can just download the standalone version of SkyDrive Pro for free via the link as provided in the previous reply.
If you want to use other Office 365 plans, like Office 365 Small Business Premium, you can take advantage of the included Office Pro Plus licenses to download the Office Pro Plus. But you can still use the current Office 2013 without installing the Office Pro Plus.
For your reference:
Office 365 plans for business
Thanks,
Cherry Wang