You did get the roof leak fixed first, right?
If I was given this job I'd never just fix it, I'd remove all the trim and take all of that stuff down including the ceiling. Just look at the condition of the ceiling, it's all cracked and looks like there was old oil based paint on it and someone tryed to use latex over it and it all alligatored.
I'd also be looking for mold, mildew, roted wood. That's not a one time small leak causing that kind of damage.
With the plaster and lath down you could add blocking at the tops and bottoms of the walls to stop air flow, add outlets, foam any holes were wiring was run and insulate to todays.
Replace the plaster with sheetrock, May have to use lattice strip to bring the rock out so it will be even with the window jams.
Once done you would have a nice flat smooth trouble free wall to work with from then on.
I have no clue what that walls made of, plaster is gray or white from the lime in it.