You can use any cooler you feel comfortable with.
But,
You will have more predictable performance if you shoot for a stable trans temp. About 190*F is a safe target.
You'll want adapter fittings to go from the factory trans lines to *insert choice of fitting style here*.
I bought adapter fitting from Black Market Parts (you need 2) -->
http://www.bm-parts.com/index.php?ro...&product_id=59
You should also use a thermostat. This is the recommended one -->
http://www.improvedracing.com/oil-th...65f-p-423.html
Then whatever you wish as a the cooler.
Some good pics here -->
http://www.e90post.com/forums/showthread.php?t=1297436
You'll also want to buy the coolant crossover pipe from the manual trans 1er. PN 17127548229