I am planning to replace my unreliable AC system with a Carrier Infinity system, furnace (120k BTU), AC/heat pump up to 19 SEER, 4 zone manifold with modulating dampers, 4 zone controller, fan powered humidification, high eff. Filteration, UV lights. The overall installation cost is quoted as $18k.

How does this offer compare to other installations of a similar system?

You will need to get estimates for other systems in the area where you live. The rates vary all over the country. Some labor higher and some less. I would get at least two competitive bids before signing on the line.