Hi, if an old house previous owner did not stay for few years already, the pipes did not run for some time. Not sure what type of pipes also, possibly copper. Can this cause things like rust and bacteria such as Legionella, etc. ?

Have a solar water heater on the roof, not sure how long already there, surely have hot and cold water pipings, concern whether the pipes still can use or better to change. Trying to avoid having to redo piping for whole house as that will be costly.

Also thinking whether to stick with solar (but potential costly maintenance in future), storage type (but not very comfortable with potential risk of explosion if not maintained well plus right above my head in the bathroom ceiling), or instant water heater (but can't enjoy powerful rainshower hotel feel).