Goodman cracked heat exchanger

I have a decision to make and could use some help sorting out the options. Our house is a two story house made in early 1900. It was at one time a single family house which was made into a duplex which was turned back into a single family house. As a result of this, there are two furnaces in the house. One for the upstairs and one for the downstairs. They are separate systems in every way. The house has no insulation in the walls and the windows are newer but not very good. the upstairs is only about 1000 sq feet and the downstairs is probably about double that space.
The upstairs furnace is a goodman GMP050-3 and it is in an unfinished attic space with good access to it. The downstairs furnace is in an unfinished 1/4 basement and that one is a goodman GMP075-3. They both are from 1996 and have had no problems since we have owned the house.
As I typically do, I had a fall service done on both furnaces. On the GMP050-3 (upstairs furnace) they said the VEST PLATE was cracked. This is part of the heat exchanger. They explained the dangers of CO2 getting into the house and that the heat exchanger needed replaced or the furnace replaced. There are pictures of the crack below.
The quoted price for the heat exchanger replacement is $900. A new Goodman furnace with 80% efficiency installed is about 1600$. I did a price comparison with a different company and they gave me a price of $1300 for an American Standard 80% Eff. furnace. So the $900 just doesnt seem like a smart idea to me. Why do a repair on a 13 year old furnace? It just doesnt make sense when a new one is just $400 more.
My problem is that I feel like I am wasting money. $1300 for a furnace that is 80% Eff and is only heating two bedrooms, bathroom, and hallway seems like a waste. I feel like I should try to get something that will provide more than just heating. Oh, I forgot to mention that we do not have A/C in the house. Adding A/C is about $2600.
I have thought about alternatives such as ductless A/C-Heating systems, electric baseboard, or a heat pump. The heat pump is out because we are in an area with cold winters and sustained temps of below freezing are common. From what i have read, heat pumps arent efficient in those temps.
I was discouraged from the ductless systems for a few reasons. One being that we already have ducts, two being that the ductless systems arent very nice looking, and three being that they have lots of moving parts that can break.
Lastly, the energy tax credit appears to only apply to 95% efficient gas furnaces. So the 80% one I would put in would not apply from what I can tell.
I inquired about the 95% Eff. models but was told they dont like to put them in unfinished spaces because of the chance of freezing in the winter. My attic is unfinished but it is insulated. I dont know how cold it gets out there but it isnt heated.
So I dont know what to do. I know that getting the heat exchanger repaired is out of the question. But does it really need fixed?! I see the crack but it only appears to be in the front and while I understand the risk (and am not willing to take the risk) I just dont see how this crack can cause problems.

 

a high efficient furnace can easily replace your old one. It needs to been properly insulated but thats simple enough to do. look for a good contractor and get the job done right, you won't regret it. the fuel cost savings will pay for the furnace over three to five years and every high efficient furnace has a stainless steal heatexchanger, they carry a life time warrenty on the heatexchanger. just remeber that the reasonable life expectancy of a furnace is 15 years, use the fuel cost savings and put it aside for the next one

 

That heat exchanger is under warranty (20 years). Replacing a heat exchanger in a GMP is normally about a 2 hr. job.

I, nor many manufacturers, suggest a condensing furnace in an unconditoned space.

 

they are quoting about half a day of work or more to replace it. The part itself is about $250 without the warranty according to them. Thankfully I checked and found the warranty paper because then they checked on it so I can save that much atleast.
I feel like getting the part myself and doing the replacement. worst I am out is the time since I am not supposed to run the furnace anyway.
So if I cant put the high eff. furnace in then my choices are fix the one I have or pay the $1300 or more for a new furnace?

 

I think their price for repair is crazy but I don't know labor rates in your area. Here, 2 guys for 1/2 day would cost around $500-$600.
I'm not saying you can't install a condensing furnace in the attic but I've worked on many which were frozen.