Mike,
You have it right, as far as the AO design. One can put together a machine such as you described, and it may work quite well, but there are a lot of other things involved in designing an extraction system.
Other than the correct components being selected, you have to choose the right hardware. We use ALL stainless steel hardware in our electronic truckmounts. We select all components on the basis of their quality, and how perfectly they fit our design of the systems themselves. We choose to go for quality to reduce the chance of any downtime, because downtime is money and time lost. I'm sure Ed does the same thing.
Believe it or not, the design of the recovery tank does have a positive affect on the amount and strength of the vacuum system itself. It's called "wind tunnel testing", and we do that here.
The machine should be well constructed, able to withstand abuse, be easy to use, clean out, and service. It should also be easy to transport from one job to another, and when being used for cleaning, whether inside or out of the vehicle.
Let us all know how the machine works out for you.
Best regards,
Bob Savage
SAVAGE-1 truckmounts
PS - I can show you a picture of the first electric truckmount/portable that we built in 1986/87 if you'd like. We still have it, and the van, although it looks a bit rough on the edges from all the use. And YES, it still works as a backup van if we need it, and does rugs here at our shop. Some of the components are 230V.