Assume that you get the new pump, or the old pump, installed and working.
The city is going to have serious heartburn about a connection of a submersible pump to their system without, and maybe even with, a Reduced Pressure Zone Backflow Preventer.
A valve separating the systems and a promise to keep it closed when using the pump is not going to be acceptable. The only thing that might be acceptable will be the backflow preventer, or physical separation of the plumbing in a way that is not easily restored.
I would install a used pump only after checking its performance, and if I were capable of removing and replacing it without having to hire a pro. The cost of a pro fixing the system will be more than the cost of a pump.