A fixed gantry (or "bridge") design like you've drawn is more rigid than a moving gantry, and it can be made very massive, since it doesn't have to move. The "column" type mills are typically small machines; larger ones are usually massive castings in a "C" configuration. But mills like the one you've drawn are becoming more common in industry. They are easier to build if you don't have an iron foundry, and work well if attention is given to rigidity in all parts and assemblies.
You say this is a "CNC/manual" mill; I'd suggest you choose one or the other. Ball screws are used for CNC, they're not used for manual mills because they tend to back-drive - you'll need a way to lock down any axes you aren't using for a particular move. Is this machine supposed to have dual spindles and another axis parallel to Z? If so, you'll need a way to drive the two separately, this gets complicated. Also, you'll need longer rails for X, and a footprint about twice as long as you've indicated if the tool is supposed to reach the whole table area.
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