I picked up an older model (ADB, the mid-80s to late-90s Mac keyboard/mouse connection standard) KVM box, for an arcane purpose, on (I think) eBay. Used. Works fine. The arcane purpose is to try hooking an optical mouse to an Apple IIgs, which actually was the first Apple model to use ADB.
OTOH, for the cash you may be better off just getting reasonably priced USB keyboards and mice. I see your point though -- just switch the kb/m as needed.
Thanks for the offer. However, I'll buy one that has a DVI port.