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

How tight is the 10mm spec on these spacers?  I can't find metric sized materials but can find something close in imperial measurements.  For example I can get several materials you listed in .375"/9.525mm thickness.  Or should I look to build up a material that is thicker than 10mm?

Is there a specific reason why you chose 1495mm and 990mm for the extrusion rail lengths for the base? Some retailers sell extrusion profiles to "spec" while others sell at standard length of 1500mm and 1000mm.  I don't think it matters (if you compensate of other parts in the build) but just curios for those exact measurements.  Perhaps I'm missing something?

Good question.

There is no complicated reason behind those exact lengths. The profiles are normally sold in meter lengths, and the dimensions were chosen mainly to reduce waste when cutting the material.

So instead of designing everything around perfect catalogue lengths, we cut the profiles slightly shorter where it makes sense.

Using 1500 mm instead of 1495 mm, or 1000 mm instead of 990 mm, should normally not be a major problem as long as you understand where the extra length goes and compensate if needed during the build.