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.
![Big Grin](http://www.theforumapp.com/alpina/images/smilies/biggrin.gif)
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.