I'm setting up a periodic port scan and vulnerability scan for a medium-sized network implementing a customer-facing web application. The hosts run CentOS 5.4.

I've used tools like Nmap and OpenVAS, but our firewall rules have special cases for connections originating from our own facilities and servers, so really the scan should be done from the outside.

Rather than set up a VPS or EC2 server and configuring it with various tools, it seems like this could just be contracted out to a port and vulnerability scanning service. If they do it professionally they may be more up to date than something I set up and let run for a year...

Any recommendations or experience doing this?