I looked into the home built engine
driven thing a few years ago and with a Cat pump with SS water end and electric clutch
and a SS pressure regulating valve and fairly large capacity membranes the cost worked out about 1/3 of a similar capacity commercial
unit and I had complete component placement flexibility with the home built.
Another advantage with the home built is that in acquiring the parts you are aware of the supplier for spares which can be a bit of a headache with some professionally built units and you learn how the system works.
I did not build my own as I lucked onto a used Katadyne Survivor Pro and really only needed a water maker because I was going to fairly waterless places and wanted the backup ability. However, if you have the technical skills the DIY approach is a good route
to go. I have met a couple of cruisers who built their own and thay were very satisfied with the result.