If you use the MDF torsion box mounted between a steel frame like I do now you'll have a very rigid machine:

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Mine is 18mm MDF on top, 9mm MDF underneath with 3x1 PAR pine in between ... convenient sizes at the time to fit the leadscrews inside.

Part of the reason why is you can make it so the height of the bed is adjustible, so you can still machine tall parts without compromising the rigidity (due to large overhang) when machining sheet material. That is also where the Z-axis design you have now drawn with the rails on the spindle plate is significantly stronger - compare the distance from tool to linear bearing blocks.