What type and size conductors and conduit should I use to feed a detached workshop?
I don't know what the "Electro Metro" code is, nor do you mention your location, so keep in mind my answers are based on a US installation under the NEC.

PVC conduit is fine for the whole run, and the easiest BY FAR. You can use Schedule 40 for any and all parts, buried or exposed but not subject to damage. Areas like where it emerges from the ground outside you'd need to use Sch80.
Again, 18" deep (to the top of the pipe) is fine for any residential installations. Table 300.5 is the NEC section for this.

Personally I'd go with #2AL for cost savings and 1-1/4" pipe. Since you'll have a mix of 120V and 240V loads you can run one #2, one #3 or 4 neutral, and a #6 ground. They do sell 100A MHF (mobile home feeder) cable that has this combination already grouped together. Remember, this type of wire/cable cannot be run exposed. You'd have to run the conduit completely from point to point. 
You could also run the conduit to a big box in the house then change over to #2SER cable, but this takes some skill and knowledge to make the change over and splice.

I'd put this on an 80 or 90A feeder breaker in the main panel.