The other day I spoke with a company that wanted to monitor and control 400 PCs scattered around their plant. Believe it or not, this is actually a job for ThinManager!
All that is needed is a ThinManager license and a copy of WinTMC for each PC. In this case a single ThinManager Enterprise Server license would be the correct choice. You would not even need a Terminal Server. Because WinTMC also runs as a service, the administrator could use easily shadow and control each PC without even having the PC logged in.