I am going to replace the HVAC unit in my house soon...

I can install it myself if it is more cost effective/practical but I am going to get some estimates from commercial installers.

I have been looking at units online and they seem to push Goodman Units.

I assume that others like Trane or other name brands sell only to licensed contractor/installers?

Is there a brand to avoid or you endorse?

I expect many brands require a licensed installer to install them for the warranty to be valid, something to take into consideration.
What kind of warranty do most units have?

Any advice?

Some facts if you need them.

I currently have a 2 ton split unit (Condenser outside heater/air handler inside). Gas heat, updraft heat/air handler unit in the basement. I plan on going back with the same but a better size for my house.

My calculations from HVACdirect.com say 3.5 ton A/C and 55,000 BTU heater.

My house is 1200 sq ft upstairs and 600 sq ft in the basement I want to add to the HVAC. Let's call it 1800 sq ft.

Thanks in advance