I think this approach is quite dated, and it hasn't aged well.

I'm sure the vectra still works, but there are $35-$55 options available now that handle mice and keyboard at the same time. Something like this: https://www.amazon.com/gp/product/B07NZSZFL8 also has buttons or keyboard control to switch back and forth from broadcasting to unicasting as needed. This means you no longer have to worry about finding old RF mice - everyone went bluetooth years ago, even for cheap mice (bluetooth mice won't work for connecting multiple mice to the same receiver - the bluetooth connection only pairs 1 to 1). You can probably find some old mice on ebay, but there's really no reason to.

Where sometimes it is just simpler to get a 2nd keyboard & mouse for individual control, I'd recommend doing a software KVM for that 2nd keyboard & mouse - the ability to just mouse between screens is great. Plus something like Barrier is free.

I generally agree with your concerns about power. Laptops are generally good for lower power, but you can achieve the same result with very small form factor PC's. Where I dislike laptops is the screens - I think its just about impossible to have a nice looking setup spread out across 5 laptop screens. But you can mulitplex 4 PC's back to a single monitor if you want - looks better and is cheaper.

For the PC's themselves, I'd suggest looking at virtualization 1st anyway though. I don't see any reason for 5 physically separate PC's anymore. You can use multi-desktop virtualization to split your PC into 5, and even plug in 5 keyboards and mice if that's what you want. But it seems to work best to split up your PC into 5, then plug 1 keyboard into the KM synchronizer above, then plug that KM synchronizer into all 5 of your virtual PCs. You achieve the same result, but its even cheaper, lower power, fewer cords, etc.