It's interesting you mention the ductless as I assumed this would be cheaper as well, however, the two consultations I've had with HVAC companies thus far recommended against these units do to higher cost considering ductwork for the 2nd unit is already in place. 

The first things that come to mind that I would look at is electrical(wiring, panel), scope the sewer line, insulation in attic, HVAC/duct work(also does it cool and heat the home properly), is the walls plaster or drywall, plumbing(is it corroded, leaks), etc.

To "fix" the issue of room temp A/C company said would cost thousands per room to tear into the wall to check the ductwork.

If the house has never had central AC, it won’t have any duct work, so something you may consider is going with a mini split system.

On the topic of cash-flow and investing in your property, this home was built in 1950's and the attic insulation hasn't been touched since the house originally built, so the  electric bill is quite high $400 a month in the summer because of the A\C running pretty much 24 hours a day.I went through the house and took temperature readings of the ceiling in the middle of the day and found that in some areas the temp over 100 degrees in a few areas with most in the 90s, so I knew I had to do something to get better performance from the a\c and lower my monthly electric bills.I went into the attic and replaced all of the A\C ducting as the old duct work was leaking cold air and was probably only operating at about 60% efficiency, I was quoted $2K to replace the duct-work, but I did it myself for about $400 plus a day of hot sweaty work in the attic and saved $1,500 which saves my margin and increases my cash-flow.today I am blowing in new insulation in the hopes that between the two (new ducts and 1-1\2 feet of new insulation that the electric bill will be reduced to under $300 per month = more cash-flow and margin.Now I am fortunate that I know how (or will learn how) to blow in the insulation and save even more, I was quoted $3K to have a company do it, but by doing it myself I will save $2,500 as the cost of materials was $400 from Lowe's and a few hours of my time.In all I saved $4k in doing these repairs myself, plus I will be saving on the electric bill every day going forward, and I increased the value of the property.I will post the temperature results and the monthly cost savings 🤞 in another post later.

Many HVAC contractors are skilled in modern furnaces, modern AC units, and ductwork, but lack experience in the vintage boiler systems.

I paid approx 3900 for brand name new unit in a 1400sqft prop (some of that included a bit of duct work).

Wanted to update this thread real quick.Fire was 22 days ago , and the fire investigation/estimation finally closed out.The insurance company sent me a check today which definitely will cover all the repairs and then some extra.Overall it wasn't as painful as I thought, although I lost most of my tenants as the insurance couldn't get the duct work clean till the investigation was done, which lead to them leaving (I wouldn't of stayed either).

When the tenant moved in, he bumped into some ductwork made of asbestos.  

They do not function well: access to upstairs is usually difficult, often a narrow back staircase too narrow for bigger furniture, heating and colling ductwork is not split, so in the summer the lower will freeze and in the winter the upper will be too warm (and leave a window open).