he says it's just sluggish above 4k... That's generally NOT a problem with seals... That's generally a clogged cat or stuck 6th ports, both of which are pretty simple and easy to fix.
113k on a NA rotary is nothing to be afraid of.. Now on a turbo you migt start gettin a bit wary, but that's nothing for an NA.
The signs of needed a new engine would be more along the lines of burning oil, leaking colant from the block, significant smoke at startup, flooding, and trouble starting when the engine is warm.
I'm gonna have to go against the rest here and say go for it. (after you ask about the aforementioned problems of course)
Bodywork is fixable, but check to see if the rust is more significant than he's letting on. I doubt it's ever going to be a show car, but it has potential to be a very good fun & reliable daily driver all teh way to 200k or so...