Is there anything to look for in particular as to whether the sills are rotten inside apart from bubbling/surface rust?
It's all a judgement call: unless the sills are perfect (unlikely), or completely shot (as in full of holes, chicken wire and filler), you have any number of degrees of corrosion. It also depends what you want from the car: as we've already said you could buy a rotten one and run it for a year, or you might want a concourse example.
I suppose the best advice would be to see some member's cars for a guide, but I think you need to use a bit of instinct when looking inside the sills. If it looks really bad, it probably is! Expect to see light surface rust inside, not daylight and your own feet....
On the other hand, mine was actually worse on the outside of the sills, very unusually. I'd usually say run a mile from a car that showed corrosion on the outside, so there's no hard and fast rules. [&o]
This was the outside of mine, I expected new inner sill as well as outer from this:
In fact it was very sound inside, and only needed a local repair.