The finance and the sales department need to be split from the rest of the network using routers with relevant access list on the router and permissions attached to the resources.
Design a server farm with a layer 3 switch connecting the rest of the network to the server farm. Since the finance and the sales department are most congested, it is a good plan to separate these two networks from the rest using routers. Usually these two departments have more of outgoing traffic rather than the incoming traffic due to the nature of their work, hence implementing access list and securing resources with relevant permissions will be easily manageable.
Implementing a server farm along with splitting the above said networks from the main network will ease the congestion on the main network considerably. The server farm needs a layer 3 switch that can re-route services or access to service within the network rather than the router.