Do not put in more than you can get out. Rotten wood in the sills or studs should be replaced. Masonry foundations should be repaired where necessary. If you try to do it all at once you will crack plaster. Disconnect all steam and water pipes so movement of the house won'' snap old joints.
Many old houses have crooked walls and plaster that is rough an uneven.
Wall surface replacement is expensive. Wallpaper can cover a multitude of flaws. Cracks can be patched and papered over. Where walls are crooked the effect can be dispelled by intelligent use of wallpaper patterns. Striped paper run horizontally around a warped corner will hide the tilt.