Are there any knowledgeable HVAC people here?

[forgop]

Being the anal-retentive former purchasing manager, I'm conducting an RFP myself for a new HVAC system for my 4400 square foot home. The existing central air unit is a 5T unit that is 19 years old and I have an 80% efficient gas furnace that was installed 6 years ago.

I recognize this can easily be a $15k project and there are numerous technologies available today. Geothermal will be out of the question as I'm using a $15k maximum.

The existing system provides very little airflow to the upstairs in its current state-so much so that my great room that is to be the primary living area was not even set up the entire summer after we moved here in May. I'm going to require the HVAC contractor to ensure a system is put in place through multiple systems/zoning to ensure I don't endure any more 80+ days in this room any more. The airflow to the rest of the upstairs is less than desirable.

Anyway, there will be a lot of variables that I get in the bids because out of the 16 bidders being invited (I know-I'm a penny pincher and quite the negotiator), what are the primary measuring sticks so to speak that I should be looking at other than the number next to the dollar sign?