Could you simplify this and just create a 2 level stand with the planer on the inside "shelf"?

I think I'm not understanding something.  I was envisioning a 35" high rolling stand/cabinet (20"Dx20"W) with a middle shelf at say 20" high with the planer and the top of the stand at 35" with the sander at the top.  Between the 2 machines, you have 160 lbs on the stand with most of that weight lower to the ground giving you more stability.  Rolling it around should not be a problem.  Any instability is generated by feeding heavy stock into the planer which could tip the cart toward the infeed or outfeed without added support, but if you have something that heavy, you really need additional infeed/outfeed support anyway.  Just make sure the casters are capable of supporting the whole thing.  Typically a high quality 3" caster will support 250 lbs/corner, so that should be more than enough for what you're doing.  Just make sure the cabinet built adequately to prevent racking and support that planer shelf. 

Something like this?

The center shelf is about 17" off the ground which puts the planer bed at about 20" - a little tough on the back if you plane a lot at one time. It's pretty stable, not tippy, but you could clamp a  leg to the outboard edge of the extended table if you were pushing the planer.