Sage MMS integration

We are currently quoting for a project with a client. We are building a bespoke e-commerce system for them using php and mysql. One of the requirements is that the e-commerce system synchronises 2 ways with their Sage accounts system. 1. When an order is placed on the web site then Sage gets updated. 2. When someone places an order over the phone, the web site gets updated. The stock level will be updated, but also the client who placed the order will also be able to login on the web site and see any orders that they placed either online or on the phone. Also, they want any products to be updated on the web site when they add a new product into Sage. This is still at the quote stage at the moment so we're are looking for the most cost effective solution. Not necessarily the cheapest but the price will be important at this stage. We want to be able to extend this solution to other clients in the future so we're looking for someone who can work with us in the future rather than this being a one off project. I think the solution would be to have a piece of middleware sat on the machine running Sage that does the synchronisation every 10 or 15 minutes, for example. The technology used is not important as long as it can be integrated with our MySQL database and work on the clients windows server.