Seems you got most of the answers you need; i will try to add just some information to consider before building it.
40 bar = (400 m water) is just de standing still working pressure. You must include also water hammer effect the same thats allows the watr hammer pump to work.
Here are some calculations: https://www.engineeringtoolbox.com/water-hammer-d_966.html
to help you estimate
I think you will need to go with steel for this; I have seen it work with theathed pipe just be sure is rated accordinly (welded pipe is better if you have the expertice).
I havent seen any equipment with a energy recovery that high for hidraulic although havent work with that many; i would consider it to be closer to 30%; This summer i will start my serius research on household size pelton units; please let me know your results.