Cabrit....
Think about this a bit. Analyze how much it would cost you to buy a new tranny and then install it verses getting a another car that's in better condition. Just do that first before you make any purchse. If it is cheaper to get the tranny, and it fits, by all means... it's a sound idea.
Just a note though, It's best to describe your intentions so that when your askign for help, we can better assist you. At first I thought that you wanted to upgrade from one auto to another, which to me didn't make sence. But now that I/we know the whole story, it makes more sence. Busted tranny, need a new one, ok. There are other sources for harder to find BMW parts. I came across a salvage yard that collected insured paid off bmw wreaks. Mainly major body damage. They would also have trannys there too. It may be cheaper, who knows really...
Here is one that I found on a quick google search (there are others):
Euro Depot
Dough