Hi ivan!
Since a number of these workstations will be moved (rolling trolleys) more things to be unplugged is not a case (the TP-Link unit I mentioned can be fixed to the back of the workstation and is powered from it. Then there is only the power cable between the workstation and no other cables to trip over.
Depending if you have a multi-outlet power strip on the trolleys, you could either screw the wireless repeater to it or you could use powerline adapters - these would even work when the PC is directly connected to the wall outlet. Just put a powerline adapter with has a power outlet on it's front, plug PC power cable and ethernet cable in it and tie them together, then it would be still only one cord to trip over.
Yes, I know USB 3 is under development but the impression I got from the last presentation video was that it would work with keyboard and trackball if nothing else. You mention USB 2 add in cards. Unless they are based on the Via chipset they have a tendency not to work because they are built on a USB 3 chipset and use the USB 2 pass-through feature.
The minimum requirement to be useful would be support for keyboard, mouse and mass storage devices in the new USB 3.0 driver. The new 32bit USB stack does not support isochronous transfers as required for audio or video. On my test system, the XHCI (USB 3.0) driver does not work, but UEFI installation was okay with a CD-ROM attached to USB 2.0 (the Lenovo X250 notebook has a setup setting to switch between USB 2.0 and USB 3.0 host controllers).
For old PCI slots it's easier and cheaper to find real USB 2.0 adapter cards.
For PCIe, I bought a "StarTech.com 7 Port PCI Express Low Profile High Speed USB 2.0 Adapter Card", which adds six external and one internal (that one has only pins) high speed USB 2.0 ports to any PCIe compatible PC. It came with a full profile bracket (which I use) and a driver CD for other operating systems. Part # PEXUSB7LP, BarCode 065030837408. It needs a floppy type power connector. IIRC, there is also a slightly cheaper version of this card with four ports (3+1).
Not all modern motherboards have PS/2 ports, many of them just have USB ports and as far as I have been able to find out you need a PS/2 port that is designed as a dual port to run both keyboard and trackball.
Some modern boards have still two PS/2 ports, but those that have only one usually support both mouse and keyboard simultaneously with a splitter cable (and a little bit of luck, that it has the corresponding wiring scheme, there are multiple versions for this task). Of course, without a PS/2 port you'll need a supported USB host controller like on the adapter card mentioned above.
Greetings
Sandra-Asja