Best to separate 3D printing from all other functions. Unless you want to print large “flat things” then the milling setup works with this. A 48″x48″x12″ won’t work well for anything. The whole setup is half the price of most printers so it’s pretty easy to justify 2 separate builds in my opinion, or mill out some parts for the Mp3DP.
Build it as small and most importantly as short as possible. There are no guidelines because it us an extremely wide open question. If you are only going to be milling 10″ picture frames make that your usable area. Unfortunately things like “I might want to mill full sheets one day” aren’t a good idea. I use that as an example because I get that a lot. If you are milling quadcopter frames but dream of milling out a lounge chair, really that is two different machines with extremely different tolerance expectations.
To answer your question about supports it depends on what you are doing, 3D milling, yes supports to add to z axis accuracy, but if you are just 2D milliing or cutting foam, don’t bother. At the same time supports are just about the easiest thing to add, you can just cut some strips and epoxy them on, or make fancy 3D printed ones. Some people don’t even use my legs they build the whole thing on a box/support.
I know it says multi-machine in my description but unfortunate that doesn’t usually mean with the same build.