I tow with an X5 4.4 with the sport/air leveling suspension, I have an open 18 foot aluminum tommy trailer. It's 1,300 lbs with a 2,600 lbs of car and probably another 500 lbs of stuff, so all in all about 5,000lbs, no problem. I can put the car on the trailer and fit it in my garage.
It tows easy, it has plenty of power and the brakes are plenty to handle even evasive stops, but... you have to make sure that the car is loaded to balance the tongue weight, the short wheelbase doesn't take kindly to the ass end being lifted, I wouldn't recommend an enclosed trailer the size needed for a car, it would be too much sail, I'm not sure I'd recommend a heavy steel trailer either...
An enclosed trailer would be nice if you have the place to keep it, but then I'd be thinking 24' would be minimum.