If it is just cold water, normal PVC pipe will do, you can opt for thicker one also.
But if there is solar heater or heater storage , then may consider polyurethane pipe.

Yup, both stainless steel and copper pipe is expensive. It is also hard to work with, the plumber will charge more because difficult to turn the steel/copper pipe around, need to cut and install elbows, PVC also need to install elbows, just that it is easier to glue and cut. PU pipe is easier to work with, need only the clamping tool.

PVC pipe joins have no problem handling the pressure, just make sure you use the correct thickness.

By the way stainless steel/copper pipe may have knocking sound when working with pump.