Only realised today that the house I'm interested in is already rendered on the inside.
Am wanting to do outside as well but wonder whether this would be the fast-track to massive rising damp?
Absolutely no reason that you can't - exterior is a separate skin of brickwork. Providing that the weep holes aren't filled or the dampcourse isn't breached it should be fine.
I have render inside as a feature matching the render outside - no issues with this. (it is actually an internal wall on both sides, but rendered only on one)