Many

" As a result of a remodel done by a prior owner in the 1990s, the house has a central air and a furnace, but no air conditioning."

That statement is in conflict. House has central air but no AC.

Start with a load calculation for correct sizing.

To your questions.

1. No. I would suggest an 80% eff two stage var speed furnace correctly sized.

2. SEER is all about savings. For LA, I would want 15 SEER minimum, your choice though.

3. 75 Db is relatively quiet. No higher though. You want a condenser that has a compressor sound blanket for noise control.

4. Good ductwork system is all about size, design both supply and return plus insulation qualities. How many returns do you have on each floor? It will be difficult to have consistent temperature from floor to floor with just one system unless you add a zoning control. Of course that would come with a cost. Do you have damper control on the supply trunk lines between the floors? Always suggest a dealer thoroughly inspect ductwork system.

5 CFM speed adjustment is in all furnaces whether single stage or two stage. I recommend a two stage furnace. Of course you will need a true two thermostat for best operation.

6. I always recommend a pleated filter media cabinet for indoor air quality and protection of blower motor and evap coil.

IMO

I assume the duct work in the walls is made out of sheet metal. There is no need to change it provided it is of the proper size. Any flexible duct work under the house should have a minimum of R-8 insulation and be sealed.

FYI Bryant and Carrier are the same equipment with different labels. The Bryant Evolution is the same as the Carrier Infinity.

Thank you for the responses!