A friendly acquaintance of mine does custom metal fabrication. I'd be curious to know what you think of this
http://www.brooklyncustom.com/prototypes.html
The cart was commissioned by the US military and was tested in active duty. That leads me to think it would be very expensive, but you might be surprised. Reach out to David if the design invokes your imagination. It can be dragged up stairs, but if I remember correctly the recon team gave it a whirl by tossing it out a high story window, and it didn't pass their final tests.
I'd be curious to know from your perspective which features translate to a suitable mixer cart