Are you planning to cut this deep with a milling type cutter ? Or to shape the outside of a job by swarf cutting ?
A 6" deep cut could start some serious vibrations, even when done in stages, and the vibrations will not just be in the tool, but the wood too. If you can get a good finish on the sides of the cut I'll be impressed. It's also going to get very hot in there.
I use 20mm tooling often, though only 100mm long, the best cutting are the serrated edge roughers. Something to bear in mind is the work holding, you will need to feed fast enough to prevent burning, a decent chipload is essential, and this can put quite a strain on your workholding. You also need to think carefully about what happens to the pieces you cut off, a ripper cutter will grab them and launch them out of the machine or worse drag them into the toolpath. Both have happened to me when cutting 40mm plywood.
Weight and rigidity will have to be high. My commercial router weighs 3.5 tons and it still vibrates.
You'll need a big dust extraction system. The dust hood on mine takes a 12" flexible hose and the extraction motor is 10hp. The vacuum pump is 10hp too. This all adds up to a lot of heat and big electricity bills, it's noisy as hell.
It might be easier to buy an old Biesse or Morbidelli and increase the Z travel than start from scratch.
Good luck with your project.