The previous owner had all the insulation covering the window. 

You might want to considering adding a layer of foam insulation before putting on the new siding.Be careful about the roof tearoff schedule.  

There is no insulation on the floor and the vapor barrier also does not go all the way to the exterior of the house on the side with the issue. 

It is what compensates you for and insulates you from the risk you are taking on by putting your capital at risk as well as the work (time and effort) you are putting in.

Putting another layer of drywall or sound insulation is not enough.

You might need new wallboard, studs, insulation and more.2.

There is a level of insulation from the rest of the country.  

If they are, or it can be achieved relatively cheaply (and there are already separate electrical entrances), look hard at replacing the boiler with three smaller units which will allow you to put the utilities into the hands of the tenants.3) Get the sewer main scoped to see if when you will need to replace the clay and cast iron (if it is still in descend shape, you might be able to sleeve it to extend the life).4) Window air in each unit, give the impression that the building envelope was never updated (ie. no insulation, just one or two layers of lath-n-plaster).   5) The building is also most probably balloon framed.  

- Insulation may increase if you are gutting exterior walls and if Stoughton has a stretch code.

It won't take much to fully insulate it and put in drywall and such.