I've found virtualization to be a wonderful solution for most clients that have specialty software. You can do a P2V migration of the existing system, disable/remove all software and services that aren't needed, set up a share on the host to hold all data, and leave networking disabled if it's not required for the function of the software, or if not run a firewall on the host to only allow the minimal amount of internet access required for the software to run. Create a snapshot of VM in case you need to roll back.
In most cases I set my clients up with Virtualbox in seamless mode with a link to their app on the desktop. They don't even know there's a VM there.