Plumbing isn't something that can be done by just about anybody. It takes a number of things to complete properly. Probably the most major would be the understanding of what pipes and fittings to make use of when. Failure to do so can be catastrophic.
One thing that should be taken into consideration when choosing the right pipes and fittings would be the environment the actual pipes are going to be exposed to. Particular materials from which pipes and fittings are manufactured are not able to stand up to some things. For instance, a plastic pipe won't be able to deal with the same quantity of weight that a metal pipe can handle if it's buried in the ground. A metal pipe is also usually able to better manage the stresses of cooling and heating better than plastic since it takes much more extreme changes in temperature to result in sufficient expanding and shrinking to cause the pipe to crack when it's made of metal than plastic. On the other hand, plastic is a lot more malleable than metal so it is simpler to get it to fit into restricted spaces as you would be able to cut it to the right size if you needed to.
One thing you do have to bear in mind, regardless of what pipe and fittings you end up using, they'll have to be replaced at some point or another, no matter what you use. Ideally the owners of the property that needs to have its pipes and fittings changed have the sense to have a qualified and licensed plumber replace them instead of trying to do it on their own. When they get it wrong, and you may want to alert them of thins, they may end up getting huge water and electricity bills because of leaks caused by improperly installed pipes and fittings.
Utilizing the proper pipes and fittings in every job is as important as being certified as a plumber. Do not take shortcuts, you will simply wind up coming back to do it all once again at your own cost in the event that issues go wrong.