I need your opinions and advice on evaluating HVAC contractors and their proposals. We need to replace our forced air gas furnace.
The house is two story, almost 1800 sq ft, and is located south east of Tacoma, Wa. It was built in the mid 90's. I believe it is insulated consistent with govt standards that were applicable at the time. The original (existing) furnace is a Goodman 80% unit. It's gross input rating is 45,000 BTU/Hr. The combination of the furnace and insulation seems adequate. Temperatures throughout the house have been reasonably consistent. Even on the coldest days, the thermostat setpoint is maintained. The system seems to cycle ON-OFF a lot, with the OFF phase being longer than the ON phase. Occasionally, during the OFF phase, the house feels a bit drafty, although the thermostat setpoint is maintained. The furnace failed multiple times this past winter. We evaluated the situation and decided it's time to get a new furnace.
We contacted 4 heating contractors and obtained proposals. 2 of these contractors were referrals through a service provided by the gas company. All 4 briefly inspected the existing furnace and asked a few questions about the system layout. None of the contractors performed a detailed load calc - Manual J or other. I asked one of them about a detailed load calc. He said that I could probably find a contractor to do it but it would probably cost a lot. He added that it wasn't needed. Then he cited a rule-of-thumb of 28 BTU/Hr per sq ft of living space and claimed it was consistent with Manual J procedure. (this was one of the larger, more highly regarded contractors in the area.) One of the other contractors went thru the house measuring the return air registers. All 4 contractors proposed units that are much greater capacity than the original unit. (proposed units ranged 60 - 75k BTU).
After that, I performed a load calc based mostly on ASHRAE Fundamentals Handbook. With very conservative assumptions and safety margins, I estimated a heat loss load of about 35,000 BTU/Hr. This supports the original equipment rated at 36,000 BTU/Hr output. Since I have no HVAC experience, I attempted this calc for my own point of reference.

Here's my concerns:
1.) Based on the existing furnace cycling as it does, I wondered if it was slightly oversized. If so, the larger units as proposed by the contractors would certainly be oversized. This would contribute to premature failure. All 4 contractors offer only the factory warranty which does not include labor. They offer no warranty on their installation. Is this normal in the HVAC business?
2.) Is there any aspect of the as-built duct work or air handling system which would require a much larger furnace?
3.) Would it be unreasonable for me to request a contractor to install a smaller unit (than they propose) and for me to expect them to warranty the installation? Am I overly concerned about furnace sizing? Is this over-sizing not worth the worry?
4.) I requested info on single and 2-stage 80% units. My thought was that a 2-stage unit could be an improvement for comfort and a little efficiency. If the high heat rate is slightly above the existing unit and the low rate is 60 - 70%, then it would cycle less and yield an improvement. If a 2-stage unit is sized with the lower rate above what is needed, then it will never get to 100% output. This seems to be worse than a single stage. Does my reasoning sound logical? Am I missing something?
5.) Would it be unreasonable or unwise to request a contractor to install a brand of furnace that is not their usual product line?
6.) It's difficult to identify "qualified" contractors. I've been using a contractors' years-in-business as a measure to qualify them. Many such businesses in the area are only a few years old. What other aspects can I use to evaluate a reliable contractor?