One of the best investments we ever made was to build 30 carts all the same height as the lowest position for our scissor-lift work bench. This allows for one man to move incredibly heavy products onto and off the assembly bench single handed.
We used to have a collection that was arrived at organically. There were probably six or seven different heights with at least that many different footprints. They were miserable to operate.
Having all the carts a common height allowed us to double stack base cabinets.
The scissorlift benches will rise exactly high enough to slide one cabinet onto another. Sometimes we gang two or more carts in a row. The carts each have a swing up handle so moving them empty long distances isn't a back-breaker.
These new carts have terrifically enhanced our storage capacity. Owing to shortage of Mexicans in the window manufacturing plants our job sites are taking longer and longer to get to the finish line. Windows you used to be able to get in 3 weeks now take 10 weeks. The window hangs up the drywall and the drywall hangs up everything else.
The best thing we did was spend some money on the castors. This is the most important part. Without a great castor you will struggle.