Snob

I am not familiar with Lennox mdl numbers like I should. Any idea of BTU output of existing furnace?

Here is what I suggest.

A furnace like Trane's XT95 40 KBTU size with a two ton 13-14 SEER AC condenser and matching coil. Add a whole house pleated filter media cabinet. That furnace is a 95% eff condensing model that will require venting to outside.and has a high eff blower motor but not var speed. Excellent choice for your location and home's size.

BTW, I would bet you are oversized on old furnace.

IMO
Good Luck!

Input: 75,000 BTUH

Yep, just as I suspected, way oversized.

You have replaced the parts on the furnace that often fail. The furnace may last another 5 - 10 years, but there is no guarntee. The AC tends to have a shorter life since the condenser is exposed to the outside elements.

How harsh are the winters in Detroit? How much insulation do you have in the attic and the exterior walls? How tight are your windows? These would determine the sizes of the furnace and AC. A load calculation would determine the heating and cooling requirements.

Carrier/Bryant and Rheem/Rudd are also good brands, but the smallest furnace may be a 50 or 60K BTU size.

Winters have avg in the low 20's -low 30's with. Attic has R-19, I do need more installation in the attic ( got a quote to add R-38 for $1380) unknown what is in the walls. Single pane windows have no noticeable leaks.

answer is depends.
single pane windows allow alot of temp transfer.
houses leak in unexpected places.

your unit is at the end of its useful life.
a new unit will be more efficient and provide
some savings in operation.

before you add insulation in attic you should
air seal. openings on attic floor/ceiling of
living space allow attic temps to enter the house.

investing in a blower door test to know where to seal the house leaks, and a duct test to know where to seal
the duct leaks will improve the comfort of the house.

once leaks are sealed..then blow/install insulation.
it is much easier to heat and cool a house with few
air leaks than a house with lots of air leaks.
and why pay to condition air that never makes into
the space where you live?

air sealing/duct sealing are two things that
quickly pay for themselves & make a big difference
in comfort & utility costs.

best of luck.

The problem with replacing all your equipment is that it's expensive. You are talking around $5k.

Your energy savings will probably be about $300 a year, so it takes about 14 years to get your money back.

I did an energy audit and my electric company calculated that my a/c costs were about $500 per year. Even if I improve my energy efficiency by 50%, I'm only saving about $250 per year.

So, sometimes it makes more sense to insulate your attic/walls and seal your ducts than go for a super energy efficient system. It depends on your costs.