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I recommend this book https://www.amazon.co.uk/Handbook-Gravity-flow-Water-Systems/dp/0946688508 and look at WaterAid note attached
There are quite a few factors to consider
Pipes should be buried. If at surface, iron pipes are recommended because they are stronger and unlike PVC do not deteriorate in sunlight, but using GI pipe would be ridiculously expensive except for short gulley crossings, etc.
The size of pipe depends on the volume water required at times of peak demand. If given a schematic of the pipeline with altitudes and flows a friendly pipe manufacturer may advice you on pipe size. They certainly will have head loss charts for different pipe sizes that will enable you to do this. Otherwise input the data into an online tool like this one https://www.engineeringtoolbox.com/hazen-williams-water-d_797.html
Consider need for break pressure boxes (to avoid pressure exceeding pipe limits) , placement of outlets at high points to break air pockets and the placement of storage tanks (which will also act as a break pressure tank)
Good luck
Ian Neal