Hardware for non-standard thickness doors is available, but will eat up any money you saved making the door. Door knobs expect a door between 1.5 and 2 inches thick. So if you want to make it out of 15 mm plywood, you would need a layer 3 layers thick on both the hinge edge and the doorknob edge to use standard hardware.
This would be a non-trivial weight door if you went with 45mm throughout. Plywood is not very gracious about screws into the end grain. If you are hanging this door conventionally, I think you are going to have problems with the hinge screws pulling out.
The answer to this may be to use barn door hinges, which surface mount with either heavy screws or bolts. In this case I would choose screw hardware (see below) that matched the style of the hinges. I would suggest black iron with square washers, and black square head screws.
There are ways to make doors out of conventional dimension lumber,the plank and Z brace being most common. Do it this way, with a double layer of planks over either thin plywood, or oriented strand board. A layer of sheet material eliminates drafts through the door.
Make a feature out of an necessity. Recently I did a stair railing out of 1x material. The bannisters needed to be heavier, so I glued 2 layers of 1x together, and screwed them with brass coloured screws with brass grommets. Stained with umpteen coats of empire red stain diluted 4:1 with water soluble varnish. The gold screws against deep red glaze is stunning.