eCommerce ""Integration"" varies all over the map. sync frequency, responsiveness in customer-facing interactions, product and pricing change direction and methods, etc. Website Pipeline and ROI Consulting both have tools and experience to integrate to Magento with good references. There are others of unknown quality.
The fundamental technical expertise (and therefore, value) is on the Sage-side integration - performance especially. Tell your customer to tread cautiously, because there are lots of ways to end up with an unsatisfying integration. If customer wants to reduce the risks and maximize satisfaction, tell them you'll help them evaluate options against their needs and plans for a modest fixed-fee project. do not give away your evaluation knowledge and advice for free.