FYI, I looked at the info regarding building a watermaker. It was a good start, but it certainly does not provide enough information. It describes a pump for the watermaker, but it does not describe what kind of motor to attach. it is going to take a while to get that figured out. As for the gentleman above who said he made a watermaker for 700 dollars, I would love to know how. The high pressure pumps alone cost over 1000 dollars and the membranes are expensive. I would certainly like to see a breakdown of costs and how he did it.
I doubt Brent Swain will provide specifics as he has described his method in a book that he sells. But as he hasn't got back to you, I'll tell in what I know.
Major qualification: I am thinking about this so I did a little research - I have never made one.
Most people who claim to be able to make a watermaker for a few hundred bucks use a pressure washer pump. I don't know which ones are suitable and will tolerate salt water. Sorry!
The other expensive part is the pressure vessel (The cheaper one are only suitable for lower pressures i.e. not desalination). I can only imagine that, to keep the price down, you would need to make one. Not sure what from. You can buy a length of 316 tube for $100 or so but I haven't looked into pressure rating. Don't think about PVC as even schedule 120 isn't strong enough. CF tubing would probably work but is expensive - you might as well spend the $500 or so for a ready made GRP vessel and save yourself the problems of making end cap(s), etc. This is one area I would really like some information on if anybody knows.
No way that I know to avoid buying a membrane - shop for the best deal but make sure it's for desalination.
To keep the price down, drive the pump off of an existing engine - auxiliary or generator - as described. If you want to drive it off of an electric motor, the pump manufacturer can probably advise on the power you need but I haven't looked at that. If so, make sure you have sufficient electrical power to run it.
You don't actually need the vacuum gauge or flow gauge as long as you clean the filters and membrane regularly. Really nice to have them though.
As a project, it doesn't seem too difficult but it's not just a case of buying the components and bolting them together and don't know of a step by step how to. At a minimum, you are going to have to figure out and fabricate a means to drive the pump. If you aren't comfortable with that you might be better off buying a ready made watermaker.
I think to create a system with all new, high quality parts is going to be $2K or so.
Hope that helps a bit.