How very odd. Just upgrading to RES 4 would solve your PCI issues so long as you install up to a certain version... 4.10 to be safe, as I know that is still PCI. The WS4's work on RES 4. Last I checked the upgrade from RES 3 to RES 4 was maybe a few hundred... I want to say it wasn't even that much. That is just for the 3700 activation code of course, not including the labor they would charge for the 'programming', but by the sounds of things you could manage that yourself.
If you want a simple POS, e7 is pretty simple and works just fine on a WS4... granted, you would still have to pay for the licensing of an entirely new system. I can't think of a single system that, purchased 'from scratch', wouldn't be incredibly expensive. If you're doing all the other work yourself, I would think upgrading your license would be the cheapest way to go.
If the issue here is maintaining PCI and not any real issue with the system itself, switch to stand-alone credit card terminals. Once you are no longer processing credit card information through your POS, PCI no longer matters on the POS system. If you want to freshen things up a bit, you could pull a few tricks like.. Clear all the sales and labor data. Delete old menu items after clearing. Image the hard drive to a new solid state drive (judging by the age of the system, they probably don't have an internal drive large enough to make that prohibitively expensive). Do some windows clean ups on the registry and system files. Put in a new gigabit nic with a gigabit switch (be wary of any PMS that use MAC address licensing though). Doing all of that should get the system feeling 'like new', and with the switch to stand beside terminals, you'll never have to worry about replacing the system for PCI considerations.