Hi Will --
I built this car for 1:35 scale on 16.5 mm track (even though it's shown with a G scale figure here!)
It has Kadee On3 couplers (the pins have been bent to make some levers).
The "S" and "On3" couplers are the same. One thing to note though is that the coupler boxes are fairly large. You might also consider the S scale couplers from Showcase Line -- the knuckle is the same, they mate with Kadee's, but the coupler box is much more compact:
Look for the #01295 couplers -- and, happily, they come pre-assembled!
(don't get any of the American Flyer compatible ones; those are huge)