Litchfield's methodolgy and explanations are spot on. If you follow his directions, etc. you can build yourself an engine driven watermaker for ~$1000.00 for a system that will last for several years, double that for a system made of bronze/stainless. Costs will vary based on how well you can 'shop' for the parts.
Making your own watermaker is quite easy and a DIY watermaker can have most the components 'spread-out' all over the boat if you have storage volume issues.
I was involved in the engineering of sophisticated RO, etc. equipment for many many years and will say that what Litchfield presents is 'spot on'. For simplicity and cost savings, his 'manual control method' is also 'spot on' as there is nothing better than a 'cognizant human brain' when it comes to 'control systems'.