Decorating and Painting Old Plaster Walls?

We have just moved into a old French farmhouse that we're doing up gradually. The house has a lot of genuine natural wooden fixtures throughout, such as shelves. There are some that were put up by previous occupants, but these are falling down/out. It is also a pain to hang pictures which are large, etc. as the wall just crumbles!

We have noticed a lot of the dated wallpaper is stained or peeling off and the paint is cracking on the ceilings. We have decided to decorate, preferably paint, and have removed a couple of rolls of wallpaper which peeled off intact. Underneath is not to badly marked or cracked, but there are holes from nails, etc. What kind of preparation and type of paint would be best to give a natural, light, textured effect on the old plaster surfaces? Any advice would be appreciated, as we can only imagine it will be a big job ;)