Anything over 1600 lbs trailer weight requires trailer brakes, per BMW recommendations.
The 4.6 is rated by BMW to tow up to 6000 lbs, with appropriate equipment (proper hitch, trailer brakes, etc). European models have optional higher towing ratings, and BMW also make reference to being able to tow more than 6000 lbs in some circumstances (special trailers, etc).
I wouldn't recommend an additional transmission cooler over the standard BMW one - the standard system seems to work pretty well. Not much reason to introduce another point of failure, IMHO. The X5 had a recall once for leaking transmission cooler lines, so I would just stay away from them.
The top towing expert here is withidl, one of his several posts on his Airstream towing experiences is linked below.
http://www.xoutpost.com/showthread.php?t=640
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2007 X3 3.0si, 6 MT, Premium, White
Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue
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