I understand what the OP is talking about. One of the misconceptions that people seem to envision is the group showing up to the armorers locker and requesting a plasma rifle in the 40 watt range. I envision a gun safe with 5 or more hand guns and a stack of loaded mags laying in there. I would envision a situation where the group somehow felt they need to be armed quickly (apply tinfoil here). Of course Murphy's Law dictates that you (the armorer) are not there.
Now everyone runs to the safe, reaches in, grabs a pistol and a few mags and runs off to their pre-determined defensive position. In my family that would entail the middle of the liviing room, running in circles.
In envisioning that rack of handguns my choice would be the G19 with a pile of G19 or 17 mags (mix and match ok). The reason being there would be no "looking for the pink one" or "I like that one, but I don't like that one", no numbers or use of braincells at all. Reach in and grab one, they are all the same. In my family the mix and match mags might lead to confusion so I am revising it to G17 mags only.
My experience is that the G19 is the easiest pistol to shoot, new or experienced with a minimal amount of training. My wife can take mine off of my dead body and use it effectively, or vice versa. (Really it would be off of my body, if she remembered to look). My kids can handle it and within a few minutes I am sure we could get Uncle Cletus up to speed if he were to show up. The lasting safety lessons would be easy, don't put your finger on the trigger, the kids already know that one.
As far as cheapest, I am not sure the G19 is available at any discount prices, even used. Mags are plentyful and reliable, size is right. The manual of arms is reliable and consistent. I am done.