I had a wonderful stay at Simon's flat. It's located 5 minutes away from Sacre Coeur and very close to 2 metro stations, but the street itself was calm and quiet. Though the flat is a studio, the use of space is very innovative and I never felt cramped. Simon has equipped the apartment with everything you could possibly need to enjoy Paris like a local and was very detailed in his instructions provided.
Yes, Montmartre is on a hill and there are a lot of stairs to get to the apartment (though you can find longer paths which avoid the stairs without much difficulty). Overall, a very charming flat in a very charming neighbourhood, and definitely worth the workout from the all the steps!