IMHO if this is to be used by 'anyone' not just the technically minded. The program should be able to take input in either format and produce output in either format. As an example look at this page:
LMNO Engineering Darcy-Weisbach
If you were just looking for opinions on what units to use here are what I use most of the time:
Water pressure = ft-H2O (feet of water)
Water flow rate = gpm (US gallons per minute)
Temperature = °C (degrees celsius)
Power = W (watts)
Thermal resistance = °C/W (degrees celsius per watt)
Air flow rate = cfm (cubic feet per minute (ft³/m))
Personally I can do whatever conversions are necessary. I agree that consistancy is a good thing. Everyone using the same terminology. Maybe there could be a page with definitions and conversions. Just a thought.
Either way this was a great idea. It will take a lot of hard work but I think the results will be well worth it.