First up, I don't think anyone here would advise you to GET a bigger case. Most of us do tend to stress that you START with a cab that's oversized when working things out on Modulargrid so that you can easily pick out what you can exclude. But buying a ginormous cab at the beginning and then populating it has an inherent risk of creating "too much", and it's NOT what we'd suggest. Last thing you'd want is to drop several grand and then be confronted with a large, confusing maze of patchpoints, knobs, and sliders!
Really, for a good starter, you should be able to cram everything required into a Mantis or similar sized case. Want more? Just add a second Mantis and the Tiptop hinge kit to couple 'em. Using a method of THAT sort means you can grow the synth over time and as needs dictate. But start with just ONE Mantis crammed with the modules for your "voice" and "modulation", so that by the time you feel that you need cab #2, you're very versed in what you DO need.