If you have a significant leak, it won’t be too hard to figure our where it is - trust me on that. Have you ever seen a water distribution line leaking before?
How big is the park and how big are your mains? Do you have stop boxes at the valve? Do you have meters? There might be alot better investments to make, depending on the overall situation.
I have experience with GPR and Schonstedt meters. Mixed results depending on what you are trying to accomplish and your site-specific situation - fill, metal, pavement, interference, etc…
Might try to get the salesman to demo the Aquatrac and see for yourself. Or try to rent one first.
Once you get everything located, bury a tracer wire so it is easy to find later. Tracer wire should always be used with PVC, and I am only a little surprised that it was not installed in this case…was it?