Hi, It sounds to me like you are almost out of the woods!
"Ex Fortis Fils Dulce" without googling I'm going to guess that this is roughly "how sweet it is to have an empty nest" ?
Anyway to get to your questions. I know you are in the final stages of the planing process because I was there not too long ago. As for Stainless vs Bronze vs Brass I think the discussion could easily get contentious but it really doesn't have to. For us, and in our particular circumstances, we were able to eliminate brass. Brass and saltwater are too easily susceptible to electrolysis corrosion
for my liking. Of course there are ways to mitigate that to a large extent but our budget
allowed for the higher reliability
of Bronze or Stainless. So with those two choices we decided on Bronze, only to avoid the possibility of crevice corrosion
. Possibly I should actually say the "myth of crevice corrosion" because I've never actually encountered it in the dozen or so systems I've had apart at one time or another.
As for membranes and pressure vessels, I google
around from time to time and I always find that American RO comes up the winner every time for both. They are vendors of filmtech brand. Possibly you could contact filmtech directly, explain that you are in the UK and are looking for the equivalant vendor nearer to home?
So where do you go from here?
Take your gal per hour number and talk to your filmtech dealer. Once you have your membranes/pressure vessels planed, talk to your giant pump (or whomever) dealer about a pump that will meet your needs. Next, you put a pulley on your generator
front end that delivers the right RPM
to turn the pump at the desired RPM
Bada Bing Bada Boom! Ample fresh water
I hope this path helps lead you out of the woods.