BUT more importantly we need them to be able to enter their own orders, which they can print out as needed, and then export a list of their orders for their own use in their own systems(csv format?).

We could allow customers in as users IF we could limit their permissions so they could not change items or view anyone elses information. In other words when we setup a user  we need to specify access only to certain customer (contacts) so they could not see anyones activities except their own.

Like in the customer portal. Just add the ability to create invoices.