The bench looks nice & solid, way more than what the Sherline will require from a twist/stability standpoint, but I mean a nice work surface if thats what you are after.
The metal part in first pic I'm less sure of. I think its better to mount any machine to something solid for stability & absorb vibration. Try & avoid any surface that can flex. I'm not sure but I think a lot of Sherline owners mount to a subplate of laminated MDF or plywood if they don't mount directly to the bench itself. If you want metal surface for cleanup in mind, I'd consider sheetmetal sandwiched between machine & bench?