So then chaps, now I am home I can share my plans with you

Please be gentle as I'm not used to drawing up plans and I've simply taken the dimensions of the welder and drawn the plans with the idea that the welder sits on the bottom section and I have a tray up top for welding stuff I.e. Magnets, tips, nozzles, wire spools, clamps etc. etc.
Steel to be used: 25mm square tube.
As a note before we start the dimensions of the MIG welder are:
L 500MM
W 280MM
H 420MM
So here's the base...
Although not included in the drawing I will put a few cross bracing lengths of steel tube to make it sturdy and to give the welder a place to sit obviously.

Once the base is made I can then build upwards...

The castors I am getting from an abandoned and destroyed shopping trolley. I would have gone with NickW's idea of using the complete bottom section of the trolley but whilst that is an excellent idea I really want to build it myself so I'm going with the above plans.
Is anything you've seen here fundamentally floored? Is the steel thick enough? What id you think?
