The X5 is an excellent tow vehicle, due to the handling, stability, and very short distance from the rear axle to the hitch ball (which means that the trailer doesn't tend to wag and pull the vehicle side to side). All recent models are rated to tow 6000 lbs. Good towing stability will also depend on how much weight % you have on the hitch ball; your trailer has to be loaded correctly to be stable.

At that towing weight, recommend you use the OEM hitch and wiring connection, whether you have the dealer install it or do it yourself. Doing it yourself, you likely need to have the vehicle flashed (computer update) so that it recognizes the tow module, so you may just want to use the dealer for all of it. The hitch doesn't tend to be cheap, you are likely looking close to $1000 (round figures) installed unless you have a relationship with your dealer and they are more reasonable.

You will need trailer brakes at that weight. The BMW wiring connection doesn't include the electric trailer brake controller, but it is not that complicated to have it installed, there is a connection point in the vehicle (you still need to run a control wire up front).

The 35i would pull that weight fine. If looking at a used model, the 30 or 3.0 non turbo will pull it, and will be fine on the flats, but you will notice the weight on long mountain pulls. Either the 50 or 35d would also pull it very well, with the benefit of either faster acceleration (50) or better fuel economy (35d).

There are lots of posts on this E53 board, and on the E70 board, relating to towing, hitch installations, and wiring.

Good luck

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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