20 years in a grade 2 18th century (Bath) stone 3 storey house here. No plans to move...The house has no foundations to speak of, but we figure that it's managed to survive enough floods and droughts to survive another century or 2. I've mostly cursed the 1960's bodges and the appalling way our trades hacked through floorboards to rewire and replumb in the 90s as if we'd never look at them.
I love 3 storey living, because the kids can play up in their rooms and we can have an oasis of calm downstairs (or vice versa).
Hiscox have been a great insurer for us.
Get a proper survey...get a surveyor used to that sort of building so that you can have a proper discussion about what needs fixing. If you plan to use the chimneys (we've just used a wood burner this Autumn, no need for central heating), get a sweep to take a look too. We were lucky that, when we had a survey done for woodworm etc, the guy observed the beetles were probably last active in the reign of Queen Victoria.
Alterations are possible. There is no consistent rhyme nor reason to what is permitted (but our local planners have been letting houses with "unpermitted alterations" from decades ago do all manner of things).
Secondary glazing etc is permissible, as long as it leaves minimal mark on the original fabric.
I love my big thick external walls and high ceilings. I love lifting a carpet or removing some wallpaper to fantasise about the lives earlier residents had.