Load calc for correct sizing.

Single stage or two stage

Var speed or high eff blower

Efficiency high eff or 80% eff

Warranty

There you go.

IMO

I'm seeing more customers asking if their furnace or a/c dies after hours are you going to be available? A fair question IMO

State of CA requires a duct pressure test prior to replacement on furnace or heat pump. This is a good thing as the DOE says you will loose between 18-42% of your energy in duct losses. Google "DOE Duct Leakage"

Duct repairs are costly. My system was so poorly done I pulled all the ductwork out of the house and went with Rinnai Energysaver DV wall furnace, which may be a good option ofr you and Mini split heat pump. Not saying you should do this, but examine all of your options. I'm very pleased with the comfort and economical operation.

What tigerdunes & jackfre stated...

Here is a link that might be useful: Load calc for correct sizing.