Thanks for the comments...
I am thinking that I should ensure my machine is capable of 3D, I actually have drivers for five axis two of which are 3 phase. I will be using one of the 3 phase drivers to drive my washing machine 'stepper' for the long X axis. As far as I can see being '3D ready' just requires that I have a controllable Z axis and a usable clearance under the gantry.
Mark, I think you have filled in my missing link with DXG2gcode! Thanks.

Of course I dont have to stay with Linux but I thought I would give it a try (yet another part of this project that I, so far, know nothing about!) Steve, I see a number of references to people using CamBam.

Most of the work will be basic cutting of sheet materials, ply and MDF etc, and probably making a few wooden signs for farm gates etc.
Meanwhile, back to the workshop...