I would probably use nagios with passive checks if I could not set a nagios server on the remote network.
On enterprise server hardware you can use tools like the insight manager (HP), opemange (dell) and have the passive checks query those tools. check_openmanage does that for dell servers and check_hpasm for HP hardware. With those tools you monitor all hardware in the servers (except if you add other hardware yourself, obviously).
You can use opsview community instead of pure nagios if the infrastructure must be managed by people without linux/unix skills. Opsview is nagios with a nice interface, but all the plugins for nagios just work with opsview. Just download the vmware image and start monitoring from the web interface. It is a great product. If you need to burn cash for support, you may also get it.
nagios passive checks docs