Hi Stuart - So to review. 1) your base and gantry are fine. The 200x200x6 is on the money for the gantry. It took me 4 months of analysis to arrive at that point, Maximus is 210mm high and 175mm deep by 6mm thick. If all of that gets executed properly its a great machine. 2) The z axis needs some deep thought. A small hobby mill is about 5N/0.001mm stiffness at the tool. A typical vertical milling centre is about 150N/0.001mm at the tool. Serious milling machines are way over that. If you have FEA associated with your CAD it's worthwhile using it. Maximus has been aiming at 10N/0.001mm and will probably pull up under that. Its Z is tubular and its equivalent thickness in steel is 76mm so you can now see how stiff this thing may have to be.
Learn about "whirl", it will be your limiting factor on the long drive screws... if you can measure your mills stiffness then that gives us a benchmark... or you shapeOK that would be good.
Anyone out there who would measure their machines static stiffness and publish it here would be doing us all a big favour. We need a router stiffness data base!!
Regards Peter