Do you know what capabilities the engineer outside the company network has? If he has a router or firewall that supports a site-to-site VPN, that would be an option. It would also be possible to setup a remote access VPN and use the Cisco VPN client to make connections to the network on an as-needed basis.
if the only requiremet is to grant access from an outside tech. to one or more server in your inside network, i think the better is to use a client-to-lan. The modem isn't needed.
for this you need only a vpn-device (as firewall cisco asa)
the public interface of this device will be the public peer of the VPN
you can reach more documents and howto on internet