[mitch808]

The max for your X5 is 6000lbs. The 335 is probably no more than 3500, plus the trailer.
The UHaul Auto Transporter is 2200 lbs, so very close to the 6000lb limit.

There are also 2wheel dollies as well, if you feel like removing your driveshaft and lift by the front tires.

I know for the suburban I've owned and just about any other domestic tow vehicle I've worked on, I ALWAYS install a Trans cooler, even if not towing. Heat kills a trans, real fast. A Trans cooler can bring down temps anywhere from 20-50deg, which in a craptastic 4L60e, can mean life or death for that thing.

Not sure if the X5 has anything like that. But if towing a lot or a long distance, I'd highly recommend it.

So that said, get the max tow rating you can find (e.g. 6000lbs), a high tongue weight rating, and the appropriate ball and always buy a lock, and the wiring harness to integrate your brake/driving lights. Oh, if in a hilly area, inspect your brakes and/or replace with a higher performing heavy duty pad.