Minimum item count before allowing checkout

Only allow checkout if the customer has reached a minimum item total in the basket. This would need to be configurable at the customer level and allow it to be set on the Product/Department level. For instance an item from Department A and 3 items from Department B is OK but 1 item from A, 2 items from B and 1 item from C is not allowed as products in Department C are not eligible towards the total item count.

We offer FREE shipping on an item count of 4 or more and a lot of our customers have asked if we can force their ordering department to pick 4 items so as not to incur delivery charges.