Integrating systems under Sage 50 Profesional

We currently use Sage Line 50 but our customer/prospect database and invoicing system is a separate Dos based system. We now need to integrate this into a newly acquired Sage 50 Professional. The attached document explains some of the intricacies of the way we work.
I do all my accounts work so am reasonably au fait with Line 50 but need guidance on the best ways to get all the features we have on the Dos based system. It's probable that this could be done by phone.