Wood – the name hard plastic material that you find on bathroom cupboards

In the US, most code only requires sheetrock. At a bare minimum, I'd go with green board. Better yet would be the fiberglass faced boards such as Densarmour. And then, of course, a really good paint.

Or consider some other materials...galvanized aluminum...copper...back-painted glass...all could be a nice contrast to the tile.

Some of those minimum clearances are defined in building codes and will vary from place to place. In Ohio we use the International Plumbing Code (IPC) and it does define some of these minimums and local municipalities might add additional regulations. But it sounds like you may not be as concerned with the minimum specifications, what you really want to know is what "feels comfortable" and looks/feels good. Unfortunately a question like that is tough to answer.

Luckily people have been dealing with small bathrooms for ages so there are a number of strategies you can employ and there are some specialty fixtures that can help, like narrower bathtubs (28” instead of the standard 30”), shallower vanities/sinks, shallower toilets (10” rough-in instead of the standard 12”), etc.

If you can find a copy of the codes your area uses that can be a good resources if you have the patience to read it. I would recommend an electronic version so you can search for terms.

In the IPC the relevant sections I found are here:

Section 417.4 Shower compartments

All shower compartments shall have a minimum of 900 square inches of interior cross-sectional area. Shower compartments shall not be less than 30 inches in minimum dimension measured from the finished interior dimension of the compartment, exclusive of fixture valves, showerheads, soap dishes, and.safety grab bars or rails.

417.4.2 Access. The shower compartment access and egress opening shall have a minimum clear and unobstructed finished width of 22 inches.

Section 405.3 Setting

405.3.1 Water closets, urinals, lavatories and bidets. A water closet, urinal, lavatory or bidet shall not be set closer than 15 inches from its center to any side wall, partition, vanity or other obstruction, or closer than 30 inches center-to-center between adjacent fixtures. There shall be at least a 21-inch clearance in front of the water closet, urinal, lavatory or bidet to any wall, fixture or door. Water closet compartments shall not be less than 30 inches wide and 60 inches deep.