The automatic E53 was rated 5000 lbs up to 2003, then 6000 lbs from 2004 onwards. Look for a 4.4 for any large trailer with a large frontal area. Also, my E53 owners manual did not list a tow rating. The only rating was provided with the OE hitch, and is specific to that hitch. Outside North America hitches were sold by BMW with ratings over 6000 lbs.

Considering payload, you will need a WDH, and with one, you don't need air suspension. More important to get the front axle loading right than just get the vehicle level. Payload ratings are real limits IMO, unlike the tow rating.

The E53 tows very well. The short distance from the rear axle to the hitch ball matters more than the wheelbase. Recommend the OE hitch, and if you need to wire up the brake controller the instructions are listed here.

Air forums is a good resource if you can get past those who promote a one ton pickup for towing anything.

Look at posts here and on air forums by poster withidl. He used a 4.4 for years with a 31' Airstream with great results, including over mountain passes. My towing with a 3.0 included box trailers, enclosed trailers, a mobile boom lift, and a flat deck designed for construction equipment.

http://www.xoutpost.com/942426-post16.html

Jeff

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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