I tried to reply last night, and you will just have to believe it was a great answer, but the system logged me off and didn't post the message. Here it is more briefly.
While your engineer is out on Saturday, check the electric capacity for the house. Upgrading to central AC may require a 200 amp service. If the house is still served by knob and tube wiring or even a breaker upgraded to 100 amps, your first project may be an electric service upgrade to allow you to consider central AC.
Since there is no existing ductwork you are looking at a complete system. Even though you may have heat by a boiler, a central AC unit is going to entail an air-handler, furnace, evaporator, condensor and ducts runs. Square footage is not so important in sizing as "HEAT LOAD" This is a function unique to your house and is based on volume (floor size and ceiling heights), and will be affected by less insulation and large single pane windows likely in this older structure. Your system will likely be larger than one designed for new construction of the same size.
Something to consider is whether its better to install 2-zones or a single central system. A separate unit serving upstairs and another serving downstairs has the advantage of not requiring vertical supply plenums to be installed in the the walls or in chases. In other words, one system mounted in the attic serves the upstairs zone, and a second unit serves the downstairs. Duct runs are easier, and you only heat or cool the part of the house in use. So, during the day you may run only the downstairs zone. At night the upstairs. Tends to be efficient, as well as easier to install. This is an option to discuss with a knowlegable HVAC contractor.
Once you decide on the necessary size and zone configuration, you need to decide what quality and efficiency you want to buy. This will take some homework and research. Higher quality and efficiency has higher initial costs, and long-term operating cost savings. With the above information you can put out a bid specification to several contractors. You want bids based on equal quality, efficiency, size and installation. Actual installation should be able to be completed in 2 weeks or less, but you may have to upgrade electric service first.
As you can see, its not a simple project, and one thing leads to the next. You are at least a few weeks away from someone being able to tell you what this will cost, and how long it will take. Most important, I hope you will continue to post back, so we can all learn something from your experience here.