The best choice for underground water lines is PE (polyethylene) pipe. It is run in one continuous piece meaning only two fittings. It comes in black or blue. Next best although I wouldn't use it is sch 40 PVC. 1" would be my choice.
The best choice for indoor plumbing is PEX. Run in a manifold type system; one continuous piece to each fixture; blue for cold, red for hot.
No prefilter for most softeners. No big box softeners, they don't last but a few years.