I'm thinking of doing something like this on my machine which is built for milling (see my build log) so it's quite heavy to say the least. I've not looked into plasma cutting much but I think routercnc is saying that you might not be able to get the router/mill to move fast enough for plasma cutting, since the gantry is so heavy?

Just had a quick look at feedrates and it looks like upto 3800mm/min could be required, but again I'm not sure...That should be achievable on a router with good motors etc, but maybe acceleration is going to be an issue?

The bed on my router is 18mm MDF, so not best suited for plasma cutting! However I've made the travel on the Z axis 400mm, which means I've got plenty of room to fix another bed say 350mm above the MDF one on a steel frame. I think this could be your best bet. Making it so you can raise the Y axis is also an option.

It's also just occured to me that 400mm is enough to bolt my big wood lathe to the router! So I could use it as a CNC mill/router/plasma cutter/lathe!