Bespoke OS Commerce Template with CMS integration of some pages

My client wants to integrate his existing site - simple static pages - with a shopping cart. My current developer is off sick for an indeterminate period so I'm looking for alternatives with a prospect for semi-regular future projects. He sells reports which he creates in Microsoft Word. Each product the shop sells will consist of one or more of these reports in pdf format, sometimes with, sometimes without additional resources such as image files or other pdfs. Each product will need to be available for download for a limited period which will need to be user-defined by my client on a product by product basis. Purchased products with a defined period greater than 3 months will need to generate automatic reminder emails. Non-shop related pages will need to be editable by a browser CMS such as TinyMCE or something similar. This will include removing or changing the main image of each page. I expect formatting to be controlled by site CSS - not editable by client. Functionality of bulk emailing existing customers required. Functionality for exporting of invoices required. Need a client-controlled option on the admin interface to 'Allow invoiced payment' for individual users. Then, when a customer with this activated places an order they will see an 'Invoice' or 'Invoice me' option at the payment stage. When this is processed then, the standard emails are sent out to both the customer and admin but the admin email will have INVOICED (or similar) added to the subject of the email allowing for a simple email filter to be applied using the customers email client. My existing developer was intending to build this using OS Commerce so that would be the ideal platform but I'm open to other suggestions. This client historically makes many amends and post-proof additions so please include an hourly rate for those. The project deadline is the end of October so solid availability between now and then is a pre-requisite of acceptance. Please see attached files for site design/layout - note that internal page columns differ from home page (the right hand column shrinks).