Is waterproofing included?
It should be for a shower or wet area, and it is the single most common difference between two bathroom quotes. In New Zealand it must meet the building code, so ask who is doing it and what certification you get.
Can new tiles go over old ones?
Sometimes on a floor if the existing tiles are sound and the height increase works, but rarely in a wet area where the waterproofing underneath is the point. A tiler will tell you after seeing it.
Who supplies the tiles?
Usually you, and it is worth confirming. Order around ten percent extra for cuts and breakages, because matching a batch later is often impossible.
How long will the bathroom be out of use?
Commonly several days once waterproofing cure times and grouting are counted, and longer as part of a full renovation. Ask for the days it is unusable, not just the days someone is on site.