Welcome to the forum. All newbies are invited!

Does the 5300 sq. feet represent the conditioned space only or are you including a basement? What are the areas of the first and second floors? What have you done about insulating the old part of the house and the windows?

What is your location?

FYI - general contractors know very little when it comes to HVAC. Spending money on HVAC is low priority for them. They are going to want to use the cheapest sub contractor for the work. Most people do not realize what they are getting until it is too late. It is good you are asking the questions now.

I think the furnace model number is wrong. I can't find anything with that model number. The AC condenser is a Base model. This is builder's grade equipment. Carrier has good equipment as long as you stay away from the low end models.

You also need to know the coil model and size and what thermostats you are getting. I would recommend getting 4 inch media filters.

Your house is big, but I think 160K BTU of heating and 8 tons of cooling seem over sized even for a 100 year old structure.

The first thing you need to ask is how were these sized determined. For a major renovation you should get a Manual J calculation. This is a detailed heating and cooling load calculation. If the HVAC contractor does not want to do this, then consider getting another HVAC contractor. Once the required sized are determined, then you can get to equipment selection and prices.

Is the new duct work a separate quote? Are both furnaces located in the basement?

The a lots of details to iron out. Take it a step at a time and start talking to your contractor.

Answering some of your questions: Yes, all is air conditioned. There is an unfinished basement where currently one air handler is located (the other is in the attic). The new construction and the kitchen and baths will be well insulated with spray foam, but much of the house is old and poorly insulated with some old wavy glass true divided pane windows that are pretty but inefficient.

I thought the A/C unit was a low grade unit. The new duct work is included in the quote, but I expect they will tie in and clean existing duct work rather than replace it all thus mostly existing duct work will be used.

Problem for me is that they have already framed the new addition and are ready for mechanicals to be installed. Thus, need to make some quick decisions. Sigh....

Thanks much for your kind help and feedback!

where are you putting the spray foam insulation?

putting it in walls won't work nearly as well as if you
foam insulated the roofline.
with the unvented foam sealed attic ducts & equipment
are in semi conditioned attic...and all those air leaks
between attic & living space below become a non issue
as air & thermal barrier are moved from attic floor to
roofline.
foam sealing the roof line is a much better investment.

investing in air sealing the house...having a blower door
test done & then sealing leakage areas will make house
easier to heat/cool.
a leaky house is expensive, whereas a tighter house
is efficient & less expensive to heat/cool.

what type ducts? flex or hard pipe ducting?
both should be mastic sealed to stop leakage.

I've worked on many 100+ year old homes in my
part of the south..air sealing is a necessity to keep
relative humidity under control.
maintaining 50% RH is the goal to shoot for,
and makes a huge difference here in the south.

don't make hasty decisions you'll regret later.

best of luck.