I need to replace my HVAC unit in my beach house (Bolivar Peninsula near Galveston TX). The house is around 900 sf. I was told that the type of coils is a factor to consider, related to corrosion. Does this limit my choices for a manufacturer?

The new unit should be rather quiet, as the one that I am replacing is quite noisy, perhaps because it was over-sized to begin with.

And finally, is there a way to calculate requirements if I switch to rigid duct from flex?