What should I put on a coffee cart setup?

The appeal of a cart is that it collects a whole setup into one place and can move to where the power and the space are. The failure mode is instability. A machine and a grinder together can easily exceed 20 kg, and a cart designed for plants will flex under that. Check the manufacturer's weight rating, prefer a solid top rather than a wire mesh, and get castors that lock.

Lay out the top by workflow, not by symmetry. Grinder at one end, clear prep space in the middle for the funnel, WDT tool and tamper, machine at the other end with the scale under the group. If you tamp on the cart, tamp over a leg or a supported corner rather than the unsupported middle of the top, since a flexing surface makes level tamping impossible.

Use the lower shelves for the things you reach for less often: bean storage, filter papers, spare baskets, cleaning detergent, a knock box. Keep the detergent and descaler physically separate from anything you drink out of, and label them, because backflush detergent and descaler are not interchangeable and mixing them up damages machines.

Cable and water management is what separates a nice cart from an annoying one. Run the machine's cable to a single outlet on a heavy-gauge lead rather than a thin strip, keep it off the floor where a castor can roll over it, and think about how you will refill the water tank: carrying a full tank across a room is fine, carrying a full machine is not. A jug that lives on the lower shelf solves this.

One honest limitation: a cart is not a good home for a plumbed-in machine, and it is a poor home for anything heavy enough to make the cart top-heavy. If your setup grows past a mid-size machine and a full-size grinder, the cart stops being a convenience and starts being a hazard, and it is time to find permanent counter space.