I discovered a 42u server cabinet for only £5
After managing to get it to fit into my Berlingo i took it home.
Removing the rear rails and cutting them in half gives me Qty 4 x 21u rails.
The rest of the cabinet can be used as a workshop cupboard until I have a use for it.
I've been inspired by DIY Server Rack so the main structure will be 2x4's though I will probably use mortise and tenon rather than pocket holes.
Then plywood to sheet the structure and 4 heavy duty castoring wheels underneath all of which will have brakes.
I will need to plan the desk section on top and the hideaway/fold out worksurface elements.
The height of 21u is 935mm and the recommended standing desk height for me is 1200mm
That gives 265mm for structure below the rack and structure above the rack to deliver a working surface at this height. That should be enough to accommodate the wheels and desk surface.
The rack will probably take an area similar to the original server cabinet so a minimum of 960mm wide by 600mm deep.