I don't know for certain, but they did have a 5 speed so I would guess it's the same ZF unit used in the 328 and US M3.
"Appropriate" is relative of course.
Basically any manual from an E36 or E46 will mate to the B6/B3 engine.
Cost aside, the best option is the E36 Euro M3 6 speed (expensive, hard to find), the E46 M3 6 speed (expensive). Those are pretty well documented swaps and the parts required are readily available. You can also use an E46 330 / Z4 6 speed but this is not as well documented. And finally the cheapest and easiest option is the 5 speed ZF from the 328 / US M3.
I'm personally doing a manual swap in mine. I do everything the hard way so I'm using the E46 330/Z4 6 speed swap. Just acquiring a few more parts before I remove the Switchtronic.
