I get the impression that you're going to do this work yourself. Have you actually costed it out, including the cost of your own time, plus the likelihood that these will need possibly more hardware support until they reach final end of life?

You may be better just reformatting the hard disks and taking them to the local recycling place and buying new ones.

The PCs will probably not be re-used and security concerns will most likely prevent us releasing these for onward sale. I just wondered if I can make one or more "super-duper" units out of what is available, hopefully with relatively little cost for other parts required.