Do consider to follow Leo Lichtfeld's description and plans, as they are spot on.
For long term cruising; and for commentary for a 6 year upgrade to my comments in that previous thread, consider .....
A BRONZE head high pressure pump.....
Carry extra high pressure gages, those stainless steel gages for high pressure sea water will only last 3-4 years in seawater.
Id also recommend that if you do DIY build, you also download Filmtec's Operation and Maintenance manual so that you can 'restore/regenerate' the membrane when bio-fouled and needing sanitization. Such chemicals (peracetic acid + hydrogen peroxide, and sodium hydroxide) can be 'dangerous' if you dont have the chemical 'safety' expertise. A commercial source of the peracetic/H202 is found under the brand name 'Minncare' and such 'cleaning' needs
chemical 'dip sticks' to be sure/verify that this chemical is flushed entirely after sanitization.
Placing the highpressure pump ON the engine, needs a very
strong mounting, one that can withstand the constant engine vibration. Without such consideration, your 'mounting' will be quite vulnerable to metal fatigue.
I would also recommend an automatic belt tensioner for an engine mounted pump system (McMaster-Carr, etc.).
I would also strong suggest that the chosen water feed pump to the high pressure pump be rated for 'continuous duty'
service as if the pressure pump is not continually supplied with water .... it will instantly burn out and destroy itself.
Most of the components can be found on eBay, etc. if youre patient. Typically most of the components will be found there during wintertime when 'pressure washers' arent being used.