Bill, we're happy to assist with questions in detail at 888-206-9938.
Water replacement is not required just because a salt chlorine generator is going to be installed. For pools in general, that is typically only required if it would be more expensive to fix a water balance / cleanliness issue manually than the cost of refilling the pool.
The EDGE-Series you are asking about is a popular high quality model. In choosing a model, these are the general sizing considerations: https://www.circupool.com/5-Reasons-why-you-should-over-size-your-salt-system_ep_94-1.html