The u in uPVC stands for "unplasticised". It is the stiffer material. Plasticised PVC is used in, oh, say, cable oversheaths, where a certain amount of flexibility is required in the material. There is a thorough explanation available in a good article in Wikipedia on this subject.

There are standardised products available from a multitude of home improvement outlets and builders' merchants. Reinvention of the proverbial wheel is rarely necessary.

From the water purity viewpoint, the unplasticized would be better since the plasticizers will leach into the water during service life (to what extent has been researched thoroughly). From the pratical viewpoint, you may require the plasticized, for making bends and so on.

You may take a look at for more information on water and chemicals diffusion into and from PVC (thread: diffusion through PVC foil - 0.2 mm)@ Diffusion in Polymers Platform

Regards,