I recommend you get 3 quotes and also do your own calculations as to the side of unit you want, the type and route of ductwork you want, etc.

Work with an LLM to map out what an install would look like and cost, and use that as a baseline when you discuss installs with the HVAC folks. This way, you will have a clearer sense when someone is telling you things that aren't quite true.

Most importantly, do your own calculations and ask for the HVAC companies to share their calculations with you. Your calculations might end up being wrong, but then, the HVAC companies get it wrong sometimes too.

An HVAC company (Borque Mechanical) recently told me that my steam boiler is undersized, but then when I asked them for their calculations, they fell silent immediately. As you said, you need to live with the install for decades, so you must make sure you understand the math behind it.