I replaced a mixer type shower that used our stored hot water with a shower that used an internal heating element and cold water mains water pressure, f you dont have decent pressure upstairs dont bother.
Because of the high current draw of the heater I had to upgrade the power supply cable and the breaker, the breaker is the easy bit, the cable however was the nightmare, it took me two days to do , this includes chasing out walls, fishing cables through/under floor boards, blah, blah, blah. then all of the making good again,
Eventually 2 days later we could enjoy a nice hot free flowing shower,
we bought the actual shower unit as a bargain from the Big Quaint hardware shop, the install however cost probably double by the end.
My Missus later told one of the neighbours what a fantastic job I had done, they went out and bought a similar shower and asked me to fit it for them, when I told them it would take me about two days and the cable route I would have to carve theough their house they took the shower back.