I am replacing my entire system and have my first appointment tomorrow to get estimates. I am going to get at least 3 or 4 before making a decision.

What I currently have is original to the house which was built in 1976 (35 yrs old). My house is a brick, 1 story 1850 sq ft. It has (2) 3 ton units on it with natural gas heat. I want to tear it all out and replace it with one large unit for the entire house. I'm thinking 4 or 5 ton, but don't know for sure. I guess what I was wondering is are there any brands to definitely stay away from? I have appointments with a Rheem dealer, Trane dealer and a Carrier dealer. I just want a reliable, 10 year + warranty, 16 or so seer efficient unit with natural gas heat. What is a 2 stage vs. single stage? Which do I need? I have galvanized stiff ductwork in the attic. Should I tap on to the existing or just tear it all out and go with all new flexible duct? I would like to be around the $8000 or so mark if possible. I would even be willing to remove both of the units from the attic myself if that would save some cash. Any suggestions so I sound somewhat educated when I meet with these guys? Everything is so high tech, it's hard to find just a good plain air conditioner anymore. Also, does it help with these guys if you pay cash? Should I even mention that? I have been saving up and will pay cash for it, that's another reason I don't want to go overboard, trying to stay away from any type of financing if possible.

Thanks!