Repairing Limestone Walls on an old Workers Cottage

I'm looking to buy an old circa 1900 workers cottage in Perth which was built with limestone exterior walls. They're in a bad condition with a number of large gaps, and the limestone is crumbling away. It's obvious a large area of the walls have been filled with some kind of render.

I don't know much about brickwork so I'm after a rough idea of how much work could be involved. I don't want to buy a house which could fall down in the next storm.

Would it be a case of filling the holes with mortar, or should limestone blocks be cut out and replaced with stone?

Thanks in advance.