Quote #1 does not include increasing the size of the ductwork which needs to be done because you are replacing a 2 ton with a 3 or 4 ton (depending on the quote) and a 1.5 ton with a 3 or 3.5 ton. Quote #2 also includes zoning equipment/controls. There is a VERY large discrepancy in size of existing units vs. proposed units(both quotes) Did any of these contractors do a heat loss calculation? You should ask to see a manual J, which will determine the proper size system for the home. I understand the size needs to be increased due to the additions to the home, but what is the proper size? I would not sign any contract without a heat loss calculation first. That is the basics. Post back and we can continue this. Or email me @ woody22478@yahoo.com. I can forward you info and links to help explain better.

There is NO Cadillac in HVAC. Several companies would like you to believe they make superior equipment, but with every manufacturer, the equipment is basically the same, using the same components made by the same suppliers. They may have an OEM number to make substitution difficult, but this is the extent of marketing. KISS always. A furnace is only 1/2 of the system, Cooling is equally as important as heating. Heavy advertising means less quality in manufacturing. It does not equate to quality. Quality costs money, and when advertising is higher, the quality drops. This is the American business model.

I have succesfuly used hardware cloth behind the filter media to prevent it being sucked in.

As beerman has stated, while a H/W fan and limit can cycle forever, a camstat brand or many snap disc limits will wear out quickly when subjected to opening as they should as a limit. Unlike a boiler where a limit is the operating control, a furnace limit is a safety as the temp shouldn't get that high.