The ole' lawn mower wheels is a good idea.
I thought so too !
For those that don't know, BBQ wheels are hollow thin plastic. Lawn mower wheels are solid rubber (mostly? never cut one apart) and the rim is a lot more sturdy that the cheap plastic ones. You can see this in the pictures.
I had saved a good number of castors (like on couches, sofa chairs etc) but the ones in the picture seemed to be the toughest ones I had. Using those due to the weight of the welder.
On another note.....remember the 6" steel pipe I brought home in March ? I moved some from the middle of the pile, put plastic in this "trough" and filled with water. I then put one of those trees you've seen me collecting and soak for 3-4 days. Makes peeling the bark off a snap !