Steve,
Here is what you wanted to know:
-Project is actually a retrofit job, previous design is old (not done using Land F/X) and already in the ground.
-New design is being tied into the existing mainline.
-Old system is maxed out and completely under designed.
-Client wants to leave old system in play, even though it is really under-designed.
Suggested to use the POC option and place where they are going to tie into the system. Using Land F/X will make sure the design is within operating range of what is available, the only additional step that would be needed is to make sure they have enough residual pressure to handle the friction loss through the existing mainline.
Other options could include the use of Caps for the existing zones and place the Meter where existing and size entire system so they could have a proper conversation with the client about getting the entire system up to an acceptable operating condition.
Hope that helps!