If their custom apps work with Windows XP, then they clearly do not need more than 4GB of RAM. I haven't found a program yet that does not work under Windows 7 32-bit that was written from the WinXP days.
Windows 8 on the other hand....several problems, especially with printers. See if you can still get Win7 licenses.
Windows 8 on the other hand....several problems, especially with printers. See if you can still get Win7 licenses.