Christopher Tidy wrote:

Another thing I should have mentioned is that stiffness plays an
important role in resisting constant cutting forces, as well as the
cyclic forces which cause vibration. For example, if the table of your
mill distorts due to cutting forces, you get an inaccurate component.

If vibration is a big issue for you, I'd suggest looking out for a
proper 1 ton saw. Ultimately this is a much better solution to the problem.

Re the defects OP (Bernard Arnest) mentioned, where he wrote
An engine-turner definitely told me that
he had his machine in the concrete-floored basement of his house, and
when somone walked too forcefully along the floor above, each step
would be an apparent defect. So I might want to think more about the
complexities of dampening and insulation than merely about just
blithely laying on mass?
and you wrote
I'm surprised at the engine turner's tale, but perhaps someone here can
confirm it?

Although his damping example referred to a table saw,
the machine he plans to build is a rose engine, as in (eg)
http://www.pledge.co.uk/ref/rosengin.htm or
http://www.rgmwatches.com/engine.html and in a pattern from
such an engine, just as for a moire pattern, I think the
tiniest variation can make an easily visible difference.
-jiw