Need a new HVAC unit

[Tonya Spinella]

I purchased a 2 story 3000 sqft home, and I need a new HVAC unit installed. There is some ductwork from previous owner, but one guy told me it's not useable, and that I have to go with metal. I have some estimates scheduled, but I wanted to get some advice here. On average what should I be paying?

Like I said, it's a 2 story with limited attic access, and 85% basement access. ~3000 sq ft. 

- never had a central air system before, so it would be a complete installation of units/ductwork/vents. 

- wanting to use gas heat. 

   I know I'll need 2 units - one for the bottom floor, and one for the top. 

Thanks in Advance!