Hi Ian
I am not the best qualified person on the technical side of vac motors, but I believe that the diametre of the 3 stage vac motor also needs to be considered. There are the 7 inch and 5 inch diametre 3 stage vacuums which will achieve different powered water lift, as will the set up i.e. series or parallel.
I believe that a single 7 inch diametre 3 stage vac motor will achieve about 140 water lift.
What I would also suggest is that you may find it beneficial to half the length of you 50ft hose to 25ft. The longer the hose, the more restriction is placed on the airflow which will inhibit solution retrieval. I have also found that working with 50 ft of hose is quite cumbersome, and the more twists, turns and loops you have will create more restriction than having a straight hose.
You can always buy a couple of connectors and attach the second hose to make 50ft should you require the additional length at any time.
I hope that helps.
Regards
Robert Olifent