repairing walls before painting

[hessian86]

 

Hi, I am repairing walls ahead of painting and in a few spots we have drummy render, which I have removed in this picture. I have a couple of questions:

1. The house is about 80 years old and the render on the brick crumbles like sand. Is this normal/any cause for concern, as the whole house would be affected. 
2. to replace the removed render, should I use a quick set mortar, or mix up acrylic render?
3. the render has a thin white coating on it, is this a plaster coat? And after patching the render, do I replicate this with a finishing plaster?

thanks

Christian