Most delivery problems we see are not about the cabin. They are about what it is being put on. A unit that lands on uneven ground will not sit square, and a cabin that is not square has doors that will not close and windows that will not seal. It is worth an hour with a level before the lorry is booked.
Level matters more than solid
A concrete slab is not required. Compacted hardcore, a run of paving slabs, or even scaffold boards on firm ground will carry most of our range, because the load spreads through the steel bearers along the base rather than sitting on the corners. What none of those will forgive is a slope. Aim for level within about 25mm across the full footprint.
If the ground falls away, pack up the low side with slabs rather than digging out the high side. It is quicker and it keeps the drainage running the way it already ran.
Drainage and standing water
Do not put a cabin in the low point of a compound. The floor is marine plywood on steel bearers and it is clear of the ground, but standing water underneath will rust the frame from below over a winter and will make the inside smell damp long before that. If the only flat spot is also the wet spot, raise the unit on slabs and cut a shallow channel to take water away.
Plumbed units need two more things
A toilet cabin needs a mains water connection and a foul connection within reach of the unit, plus the electrical supply listed on the product page. Get those marked out before delivery rather than after. Moving a plumbed cabin two metres because the connection was on the wrong side is a lift, not a shove.
Every product page lists the electrical requirement and the water connection type under the specification table, so you can check what a given unit needs before you order it.