re: What is a reasonable quote for a new AC unit for 1800 sq ft house?

Part of your purchase decision should also factor in estimated energy savings when selecting a new unit.

This site lets you compare your current unit to your options. Just enter tonnage, seer rating, and location (represents avg energy usage) LINK

It gives you 5, 10, and 15 year savings estimates.

It helps you decide if the purchase is worth it.

Also verify that your AC guy has assessed your air intakes and returns. More efficient AC units may need to “take deeper breaths”, which could mean in order to reach that efficiency they may need to either add another intake or widen the chase from your current intake . Additional returns could help if airflow is not properly distributed to some regions of your house.