TLDR; I really enjoyed the stay and the pictures do not do this place justice.
Yes there are stairs and it’s on the fifth floor so be prepared to get some steps in.
The hosts are very friendly and welcoming. I really appreciated them for everything they did for us- great instructions/ meeting us so that we didn’t get lost. As for the place, it is really nice- the kitchen and bath is basically brand new. The rest of the flat was very charming and had a lot of cool architectural details from old trim to old doors. I really loved how everything looked. The place is clean. For being so central, I loved how cozy and quiet it is. There is ample space. The toilet is in a separate room from the shower and bath and has its own sink, which was super convenient. The flat had lots of great natural light and the curtains were blackout so it was great to get rest. The beds were comfortable.
Thanks for the stay. We would definitely stay here again in Paris.