Be carefull should you take the plaster out. If you have plaster ceilings also, make sure you cut the plaster at the top of the wall so you don't brake part of your ceiling and have to replace it also. I find a rotozip works well. I have real rough plaster walls in part of my home. Can't wait for the day I can afford to rip it all out and hang drywall. As for the cracks I have been useing calken to fill them. It has worked real well so far. With the rough walls I don't have to smooth it out. Once I paint over it, you can't tell where the crack was.
Remove the Casings
You should carefully remove the window casings. Where there are finishing nails, pull the heads through the casings; don't drive them back out.
Then you will have the studs to support the drywall.
If it is an outside wall, you will be able to add insulation, which is probably lacking since the wall is old enough to have plaster.