Plumbing is not something that can be done by just about anybody. It takes a number of things to do correctly. One of the most major would be the knowledge of exactly what pipes and fittings to use whenever. Failure to do this could be catastrophic.
One of the things that needs to be taken into account when selecting the right pipes and fittings is the environment the pipes will be exposed to. Certain materials that pipes and fittings are made are not able to endure certain things. For instance, a plastic pipe won't be able to deal with exactly the same amount of weight that a steel pipe can handle if it is buried in the earth. A metal pipe is also generally better able to manage the stresses of cooling and heating much better than plastic as it takes far more drastic changes in heat to cause sufficient expanding and shrinking to result in the pipe to crack when it is made of metal than plastic. On the other hand, plastic is a lot more flexible than metal so it is easier to get it to fit in to tight spaces as you would be able to cut it to the right length if you had to.
One thing you do have to keep in mind, no matter what pipe and fittings you end up using, they'll need to be replaced at some stage or another, regardless of what you use. Hopefully the owners of the home that needs to have its pipes and fittings changed have the good sense to have a competent and certified plumber replace all of them rather than attempting to get it done themselves. If they get it wrong, and you may wish to warn them of thins, they might end up getting enormous water and electricity bills because of leakages caused by incorrectly installed pipes and fittings.
Using the correct pipes and fittings in every task is really as essential as being licensed as a plumber. Don't take shortcuts, you will only wind up coming back to do all of it over again at your own cost in the event that issues go wrong.