Yes, HPL means high pressure laminate.

For the 10 mm Z spacers, the exact material is not very critical as long as it is hard, stable and does not compress easily. HPL works well, but you can also use other hard materials such as plastic sheet, aluminium, phenolic board or similar.

I would not recommend MDF for these spacers, and plywood is only a second choice. Good quality plywood could work in a temporary build, but it can compress slightly and may change with humidity. A harder and more stable material is better.

When I write plastic, I mean a solid plastic sheet material, for example HDPE, POM/Delrin, PVC, acrylic or similar. A 3D printed part could possibly work if printed very solid and accurately, but a machined or cut solid sheet material is preferred.

For the steel plates, normal mild steel / low carbon steel is fine. You do not need any special alloy. Standard construction steel or common low carbon sheet steel is suitable. The important thing is that the plates are flat and cut accurately enough.

So in short:

Z spacers: hard plastic, HPL, aluminium or similar hard material
Avoid MDF if possible
Plywood only if it is good quality and you accept it as a practical DIY solution
Steel plates: normal mild steel / low carbon steel is fine