Difficulty is measured by experience and exposure. If you're used to working on Honda cars, you're comfortable with that. From my understanding, 240's main concerns are body work (drifters crashing) or interior parts. I myself am willing to pick up a 240 to try out RWD when available but I already have 2 vehicles under my belt and tight wallet and i'm only 17.
If you're self-diagnosing the Rx7 or 240, it may be difficult depending on you, but i'd got to say Civic's are the more problematic and compact of the 3. Keep in mind the Rx7 are rotary, so that's a different engine setup, i'm not sure how that differs in terms of difficulty but just read up on it online.
Whichever it is they're 80-90's cars, wider aftermarket support of course, many trial and error stories to learn from and not as difficult as new cars. Try checking out a 240 forum like Zilivia or Nissan 240sx forum, idk about Rx7 forums though.
Mitsubishi Lancer Evo's/Subaru Impreza WRX (STI's) now that's a different land to venture, my bro's STI is so confusing and we're not even done putting the engine together.