You might want to keep in mind the issue of coupler height. On3 is higher than what is commonly used for On30. Also some On30 modelers use the scale/gauge combination as poor man's On3 and mount the couplers at the On3 height.
You may want to plan to have a way to mount couplers at both heights or plan for an easy modification to allow the modeler to mount the type of coupler at the height he/she chooses.
KD-803s and the San Juan Evolution are commonly used for On3 and some On30. KD-5 and KD-26 are common HO scale sized couplers used by On30 modelers.