We're getting quotes in for external insulation at the moment (Victorian end of terrace, currently acrylic render, couldn't look much worse), and are being told that the roof will need extending. Which will mean a new roof entirely, as it's on it's last legs as it is.
First step is to check your roof overhang as some older properties aren't generous enough.
In theory you can have ANY exterior finish, I'd avoid trying to match what exists already.