Ideally before the electrical wiring goes in. That is when automation is cheapest and cleanest, because conduits and back-boxes can be placed correctly the first time rather than chased into finished walls.

Yes, though the approach changes. Retrofit leans on wireless switches and modules rather than rewiring, which keeps the disruption down. We will tell you honestly what is and is not worth doing in a finished home.

Your keypads and wall switches keep working — scenes run locally and do not depend on the connection. What you lose is remote access from outside the house, until the line is back.