I am starting a new self build in spain and I am looking to buy a small like Mayco c30 or similar. I want to use it to pump concrete for foundations walling and also shotcrete for a swimming pool if possible. what do i need to know about mixes, priming the pump and methods of cleaning the pump and piping

I would get an all hydraulic unit instead.  More reliable with less maint.  For shot-crete something with a longer/fewer strokes is easier on the hose man.  Can also use a surge-T.