Hi Jason,
From your description, I know that you want to use Office 2013 and mail service in Office 365. If this is the case, you should have license for Office 2013 and Exchange Online.
Currently, the mail service(Exchange Online) is available in Office 365 Small Business Premium Preview and Office 365 Enterprise Preview. So you can subscribe either of the two plans.
For more information about services in Office 365 preview plans, please refer to the link below :
Office 365 Preview:
www.microsoft.com/.../whats-new
Office 365 Home Premium Preview:
www.microsoft.com/.../office-365-home-premium
Office 365 Small Business Premium Preview:
www.microsoft.com/.../office-365-small-business-premium
Office 365 ProPlus Preview:
www.microsoft.com/.../office-365-proplus
Office 365 Enterprise Preview:
www.microsoft.com/.../office-365-enterprise
Try more products:
www.microsoft.com/.../try-more-products
Thanks,
Linda Wang