Bill in Co wrote:

I have two old used eBay laptops (one with Win XP, one
with Windows 7), which I rarely use..

So the Win7 one would be your "trial candidate" for Windows 10.

If it's Win7 SP1 x64 Home Premium, try installing Windows 10 x64 Home over it.

If it's Win7 SP1 x86 Professional, try installing Windows 10 x86 Pro over it.

For a "free" upgrade, the "class" of the install has to match.

It's still possible for a CPU on a Win7 box, to not
be sufficient for Windows 10, but that's the most
likely limiting factor.

Both Win7 and Win10 work with 1GB of RAM. The "kernel portion"
of the OS (i.e. squeeze out all the fat) is about 350MB or so.
Using 1GB of RAM leaves room to work.

In terms of "comfort"

1) Single core computer "works", but many common activities
might seem slow. Firefox with five processes running, railed
while it loads Yahoo News, is going to be slightly worse
than it is on Windows 7. This is because Windows 10 has little
maintenance things it will be doing in the background.

2) Graphics drivers for Windows 10 are limited by the
hardware companies. An FX5200 for example, would be running
the Microsoft Basic Display Adapter. My HD6450 PCIe video card
got one driver of merit and support has stopped. The card
runs at native resolution. When the Microsoft Basic Display Adapter
runs the show, you get 1024x768 (even on a 16:9 display).

3) A "best config" would be a quad core processor, to give some
bandwidth for the background stuff, and leave a bit for you.

You have the materials to test, but it'll be your Win7 laptop
as a (possible) candidate with no guarantees. The screen will likely
run at 1024x768.

*******

On a laptop, when it gets super-thin, there's no cooling fan.
The CPU/GPU heat is dumped into the base.

The processor then has two limits. It has a "power limiter" that
throttles if the electrical demands of the processor are too high.
It also has the usual "thermal throttles". It will allow "bursts",
so on a single threaded task (with no OS background activity),
it might seem fast. But as soon as there is background activity, it's
going to disable turbo. There is at least one enthusiast class
processor, where the turbo is set up for two cores, which allows
some amount of annoying Windows 10 activity, while you "bench"
on the other core.

With the thermal limiter, you could start a movie transcode, and
be doing 60FPS processing for the first 15 seconds, and then
the frames per second processed gradually drop to a lower number.
The cooler you keep the base, the more the speed can rise again.

If the laptop is thicker and has tradition "air cooling", you might
have more headroom. Air cooling also allows cheaper competitor
CPU choices which happen to not be as efficient. Air cooling
should allow a cheaper "performant" laptop to be built.

Thin devices used eMMC Flash storage. A device with 32GB of disk
space, stores things in a single flash chip. If the flash chip
fails, nobody is going to offer you a repair strategy (chip
must be un-soldered to replace). A wiser purchase is a machine
with at least one bay for a 2.5" drive. Which can take a SATA SSD.
And be replaced if there is an issue.

HTH,
Paul