Trisquel's default system can perfectly run on any of the computer you
describe. I have no idea of FreeCAD's requirements though: just run it and
see.
a lighter Linux like Trisquel 9
*GNU*/Linux: https://www.gnu.org/gnu/linux-and-gnu.html
OS vs Software vs RAM vs Swap, issues – please?
I do not really understand the question. You need enough RAM to comfortably
work, i.e., to have all the programs you may simultaneously need run at the
same time (including the Web browser). You need some swap too. Not much.
Just in case a malfunctioning program starts leaking memory (you want to have
the time to detect the problem, save your work, and close the problematic
program) or if you want to hibernate (then choose at least as much swap as
you have RAM).
