Does your current furnace keep the house warm on the coldest days of the year? If it does, then you should not be buying a furnace larger than 80K BTU at 95% efficiency. I am surprised that the second contractor quoted a 100K BTU furnace.

Here are my comments to your questions:

1)I think it would be a good investment to change the AC now. I always worry about a leak developing in the coil when the furnace is replaced.

2)I don't think you have to upgrade your humidifier. I am not aware of humidifiers made for modulating furnaces.

3)The is a big difference between single stage and 2-stage furnaces. I feel there is a smaller difference between 2-stage and modulating furnaces. I have a Carrier 3-stage furnace. I can tell when it is on high stage, but middle and low stages are sometimes difficult to differentiate.

4)Your prices for options 1 and 2 seem on the high side for York equipment. I am not sure I would get a Nest thermostat.

Some observations and thoughts.

Your existing furnace has about 80K BTUs output. Both Carrier quotes have quoted 100K furnace models with 96% efficiency. About 16+ KBTUs additional. Wonder why?

It appears to me if your mdl number is correct, the Goodman condenser is a 2 1/2 ton size, not 3 ton as you stated. This should be checked. BTW,
you may not want to hear this but that model is a pc of junk. I would replace it with a matching unit.

Why the need for a WiFi thermostat?

Going down your list.

I would replace the AC condenser with a properly sized and matching 15 SEER model minimum.

I see no reason why your existing humidifier can not be reused.

Full modulating furnaces adjusting to demand both up and down on the fly give you best comfort.

I like Carrier option #3 over #4. I would want a media filter cabinet.

Any hot/cold spots within your home? Now is the time to point these out for dealer to address. Home has adequate return air? Where will new furnace be located? It will need to be in close proximity to Ext wall both for venting waste air and fresh combustion air.

On Carrier Infinity furnaces, I would not use any control/thermostat other than the latest Infinity.

IMO

" The furnace is original and the AC is newer, looks like a 2002 model year. Most windows face ESE but have a lot of western sun in the summer on the front of the house"

10 year AC. Don't waste your money on new AC, especially if you live in a temperate climate. I live in CT and have a
2 ton carrier 25 years old, no problem. Secret is it is only needed 3 months per year.

Preliminarily,I would vote for either 3 or 4 but if you plan on upgrading and heating the basement you could use the 100 K unit.
In any case, you need a calculation for the required BTU.
The older houses were over designed and your 100K unit was probably also. Insist on a programmed calculation from any competent installer. Oversized single stage units will cycle more which reduces comfort level.

Most homeowners are not fully aware that basements at or partially below grade carry very low heating and cooling load.

If I was the specific homeowner in this thread and wanted best comfort and control and depending on when basement will bevfinshed and how it would be used, then a zoning control might be indicated. Of course this comes with a cost.

IMO

Joeplumb,

If you think the original furnace was over sized, then why would recommend getting a new furnace which has a larger output?

I don't think the Goodman AC condenser is going to make it to 25 years. If the OP had a Carrier or Trane condenser and was going to match it to the proper furnace, then it may make sense to keep a 10 year old condenser.

I agree with what Tigerdunes has stated about heating a basement. It does not take much to heat it, but you need a zone control if you want to keep it at a specific temperature in the winter. In the summer it will always be cooler than the first and second floors of the house.

All,

Thanks for the comments--much appreciated! Specific responses to some of your thoughts:

mike_home - "Does your current furnace keep the house warm on the coldest days of the year? If it does, then you should not be buying a furnace larger than 80K BTU at 95% efficiency."

Yes. We recently had a stretch of sub-zero temps for 2-3 days with a low in the -15 to -20 F range overnight and had no problems. I went down in the basement and took a thermometer with me, and without any vents directly feeding air in, it was 59F when exterior temp was 0. (Upstairs was set at 69.) It stays pretty constant regardless of exterior temp.

tigerdunes - "It appears to me if your mdl number is correct, the Goodman condenser is a 2 1/2 ton size, not 3 ton as you stated. This should be checked. BTW, you may not want to hear this but that model is a pc of junk. I would replace it with a matching unit." "I like Carrier option #3 over #4. I would want a media filter cabinet." "Any hot/cold spots within your home? Now is the time to point these out for dealer to address. Home has adequate return air? Where will new furnace be located? It will need to be in close proximity to Ext wall both for venting waste air and fresh combustion air."

Could be a 2.5 ton, not sure. The contractor who quoted the AC units recommended a 3 ton so that makes sense. Same person also mentioned it was a piece of junk so that's not news to me, but like hearing it from an independent source vs. a sales pitch.

There are a couple hot/cold spots which we pointed out. Partly I think this is due to the oversized old furnace stopping/starting vs. the ducting, which contractor #1 seemed pretty happy with overall (had 2 vents for each return, well placed, etc.). Furnace is located in the center of the basement but a pretty easy PVC run the 12" or so to the rear exterior. We have a VanEE filtering the intake.

joeplumb "10 year AC. Don't waste your money on new AC, especially if you live in a temperate climate. I live in CT and have a 2 ton carrier 25 years old, no problem. Secret is it is only needed 3 months per year." - Agreed, a couple years ago we only used it for 2-3 weeks. Last summer, of course, it saw a lot of use.

I'm not unwilling to spend $$ on a replacement but want to feel good about it... if there's a lot of useful life left on the unit (5+ years), then I'd rather wait. If it's likely to die soon, it makes a lot more sense to replace. I know you get a couple points higher SEER with the 2-stage AC but is it worth the extra cost given the climate here--avg high temp in July is 84, though humidity is also usually high in the summer.

Thanks again for all the thoughts.

Two stage AC condenser not required. Stay with a good high eff sgl stage model properly sized with best matching coil.

IMO

"Joeplumb,

If you think the original furnace was over sized, then why would recommend getting a new furnace which has a larger output? "

Read the entire post. Has 100K -- who knows what he needs.

He needs somebody to either calculate the requirement or experiment with his existing one to get it empirically'
This last is tricky. The way I would do it is set thermostat
super high so the furnace runs constantly and at equilibrium get the indoor temperature. Now if you assume say 80K output for the present furnace, you need approximately
80*(70-T0)/(Th-T0))
70 assumed indoor required temperature or whatever you need
Th= temperature the house reaches for full on furnace
T0 outside temperature at the time of experiment

"Joeplumb,

If you think the original furnace was over sized, then why would recommend getting a new furnace which has a larger output? "

Read the entire post. Has 100K -- who knows what he needs.

He needs somebody to either calculate the requirement or experiment with his existing one to get it empirically'
This last is tricky. The way I would do it is set thermostat
super high so the furnace runs constantly and at equilibrium get the indoor temperature. Now if you assume say 80K output for the present furnace, you need approximately
80*(70-T0)/(Th-T0))
70 assumed indoor required temperature or whatever you need
Th= temperature the house reaches for full on furnace
T0 outside temperature at the time of experiment