This article might help, as it tells you what stations are fully accessible. Not surprisingly, line 14 is the only one with full coverage with elevators. They do say that "Roughly two-thirds of Paris metro stations have escalators," though I must admit that I think that could be over-estimating the number. Despite the stairs, though, Paris underground stations are not very deep underground, so it's usually only one or two flights down to the tracks, unlike the seemingly endless depths that one has to travel down in many of the deep lines in London.
Central Paris is small, and covered by a spider's web of metro lines (including the RER). My impression was that you were never more than a 10 minute walk from a metro station, often multiple stations on different lines, actually. Every major tourist attraction is accessible by Metro, even some of the out of Paris attractions are easy to get to by train. Check out this interactive map. It shows most of the major attractions, and you can zoom in to see the nearest metro stations. Unfortunately they don't show the lines, but you can figure out the lines from the Metro map from RATP. Also, if you use the RATP interactive map, you can get a small map of the local area around the metro stations, which also lists major tourist attractions.
I used to be a fan of the Carte Orange, but then I crunched the numbers and it turned out cheaper to just get tickets, or on a heavy travel day a Mobilis for daily unlimited travel. The Carte Orange is only good for a weekly period (Monday - Sunday), and to make it worthwhile you need to be travelling more than 14 metro trips during that time across 3 different days.