There's more than one way to line a van, and it's easy to get pulled into thinking you need to pick just one. In reality, a lot of vans end up with a combination, so it helps to know what each option is actually for before you spend money on the wrong thing.
Ply lining: Ply lining is a hard, rigid panel fixed to the floor, walls and doors of the van. Its main job is protection - it stops tools, stock and everyday knocks from damaging the van's bare steel, and it gives you a solid surface to fix racking, shelving or a load restraint system into. It doesn't add meaningful insulation or soundproofing on its own, but it's the most durable option by a distance and it's what most trade vans are built around.
Foam insulation: Foam insulation - usually a closed-cell foam sheet or a sprayed-in product - is about temperature and noise, not protection. It's fitted between the van's ribs and skin, underneath ply lining if you're using both, and it makes a real difference to how loud the inside of the van is on the motorway and how much heat escapes in winter. On its own it does nothing to protect against knocks or scuffs, which is why it's almost always paired with ply lining rather than used instead of it.
Carpet lining: Carpet lining is a softer, fabric-faced panel, often used in vans that double as personal transport - a camper conversion, a van used for family trips as well as work, or anywhere you want a warmer, quieter feel over raw ply or metal. It's more forgiving on paintwork and skin if you're loading and unloading bikes or furniture, but it doesn't hold up anywhere near as well against sharp edges, grit, or repeated abrasion as ply does, and it's harder to keep clean if you're carrying anything messy.
Which one do you actually need? If the van's doing genuine trade work - tools, stock, ladders, racking - ply lining on its own is usually the right call, with foam insulation added underneath if noise or temperature is a problem. If it's a van that's being converted for living in or for mixed use, ply lining as the structural base with carpet or foam over the top gives you the durability underneath and the comfort on the surface.
The short version: ply protects, foam insulates, carpet softens. Most vans that see proper daily use are better off with ply as the base layer no matter what else goes on top of it, because it's the only one of the three actually built to take a beating.