blue MDPE for underground stuff switchin gto copper above ground. I can't remember the depth for burying a water pipe off hand but i "think" its something like 600/700mm. If you use MDPE you wont need any mechanical protection.  The other real option is underground copper(table Y copper) but this is seldom used these days. Plastic is good for underground as it wont react with any chemicals/minerals in the soil, and wont get damaged by UV rays as its underground.

You will need a DCV on it and I would suggest a stopcock on the house end for turning off the water to lay the pipe/repair the pipe, and another in the garage for convenience.

btk