Fellas.
I should start charging people for my money saving ideas. I swear to God.
Type gas furnace into ebay, there are many HVAC dealers that sell direct and will ship to your house.
Then type tankless hot water into ebay, you can get a titan shipped to your house for 228, free shipping.
We just put one of these in my buddies house, works excellent.
Now put an add up on craigslist, for a certified HVAC guy to come and install both units in your house, pay him cash and call it a day.
Any HVAC service out of the phone book makes a healthy profit on the equipment, and the labor, plus they charge you tax on both.
I bet you can do the heater, and the hot water tank for around 2 grand installed.
Buy me a beer or something when you get it all in.
Also I highly recommend getting the new heater ready for an AC unit when they are installing it.