Help with choosing proper converter amperage

I would stay with the 60 amp unit unless you are planning to enlarge the wiring between it and the battery. Typically the manufacturers use the minimum size wire that they can get away with and the voltage drop would probably limit your charging current to less that 55 anyway.

The larger units would not cause any "issues" but would not provide any increase in value either.

Changing the charger will have no impact on adding the second AC, but if your trailer service is only 30 amps you will not be able to run both at once.