I am in the process of getting a new HVAC system for a 1900 sq ft. townhouse in RI. I have 2 conditioned walls, 3 sliding glass doors, 5 windows, and a picture window. House is well insulated. I used 550 CCF of gas for the last 6 winter months (includes dryer, stove, hot water heater).
About $1100 total which I figure heat was about $700. Thermostat was set at 72 degrees (efficiency is good compared to other people I have talked to).

I now have a Lennox. This system is 20 years old but AC has a leak. Lines are in the finished basement ceiling. Not worth is aggravation. Therefore, I am going to replace AC, Furnace, Coil (20 years old system), and maybe the hot water heater.

Information I have is the following for the present Lennox system.
AC is HS18-261-8P 1.5 ton 8 seer
Furnace is G12D2-55-3 55,000 BTU at 60% input efficient
Evaporator is C16-21-1FF 2 ton
Water Heater is a Bradford MI403s6LN12, 272therms/year, Energy 215-283

I am in the process of getting 3 or 4 bids. My 6 questions are:

1. Should I get a high efficiency furnace 90%, or is 80% OK???
Condo association says I can run the exhaust for high efficiency out the back of house.

2. Should I replace the hot water heater at the same time???
I ask this question because I rent the hot water heater for $13 per month.
I do not know the efficiency of the hot water heater. If I do replace the hot water heater I will now have service from the same contractor. I will get a tax benefit. I will not have to pay the $13 per month.
Water Heater is a Bradford MI403s6LN12, 272therms/year, Energy 215-283

3. Do I understand the Goverment Energy Star Tax rebates???
$150 for 90% or higher gas furnace.
$300 for AC 15 seer (15 seer is overkill for Northeast).
$300 for Hot Water Heater Energy Factor >= 0.80
Total not to exceed $500.

4. What seer AC should I get???
Not a big benefit from the 15 seer in Northeast but, I do get the $300 rebate and will save on costs in long run. I ONLY AC 1300 SQ FT. OF THE TOWNHOUSE.

5. Will I run into problems with high efficiency hot water heater or furnace exhaust???
I currently have a 5 inch pipe that goes to the roof. This exhaust is for both the water heater and furnace.

6. What humidifier should I get???
I was told the best way to go is to get a separate humdifier and put it on the second floor. The humidifiers sold with the furnaces are not as good.

Any other thoughts would be appreciated. Trying to figure how to get best service, most comfort, and biggest bang for my buck.
Don't mind spending the money for quality. I chuckle at people who spend alot of money for their house but, freeze/sweat their butts off because their to cheap to spend for HVAC.

Thanks for your time. Great Forum!!