On the matter of casters, I had some heavy duty critters I acquired from who knows where. I needed a rack that would hold my collection of lumber. Up to around a thousand pounds. The casters could handle it, BUT the
wood (2x4's) the caster posts went through couldn't. They started moving to an angle THROUGH the wood and I could barely move the rack, when loaded. It was a matter of time and I'd have to call it a stationary storage rack.
I ended up pulling out my floor jack, lifting an end at a time, blocking both sides up (safety), and removing the casters. I replaced the single caster in each corner with three high capacity Harbor Freight ones.
To install the three casters in each corner, all I did was mount them on about an 8" square piece of plywood, and secured it to the bottom of the rack.
Four years later, the rack still moves freely, with just one hand.