We just did this update to our house built in 1919. We ended up going with a 4 ton unit for the downstairs and a 3 ton unit for the upstairs. The size I believe is based on square footage. My understanding is that 500 sq ft = you need a 1 ton unit, 1,000 square ft = 2 ton unit and so on. Apparently it's better to go up a ton if the square footage is somewhere in the middle.

The biggest hurdle to updating the AC was by far the ductwork. The ductwork in our house was added when asbestos was still being widely used. All of our ducts were wrapped in asbestos tape. I would look on Google to see a pic of what this looks like. If the house has it it will unfortunately add a lot of time/money to the project. For the new ductwork and 2 AC units we paid about 15k - we ultimately removed the old ducts ourselves to keep costs down. I shopped around for about a month and got a ton of different quotes. Most were in the 20-25k range.