I feel the PM's residence is a great part of our Canadian History. I would NOT replace wiring and plumbing. I would make sure both are in good repair and add to it but not replace it all.
No new windows. Only add storm windows. The old windows are historic.
No drywall. Only plaster and lath. It's keeping with the history. 
It should be of museum-like quality, not modernized and have its history destroyed like what seems the trend these days with all old buildings.
And anyone who thinks it should be torn down and to build a new building is an anti-historic very naive person who doesn't understand history and that the building will be valued by future generations.