Put a hitch on the 900 Turbo. I have an 88 900spg that tows small trailers without a problem. The mileage drops from over 30 down to about 24mpg towing about 1000lbs.
The 300D is way too new and nice to have an ugly hitch on it.
As far as heavy trailers, I'm guilty, I've towed a MasterCraft ski boat with a 78 Saab 99ems. The Saab engine pulled it fine, but,,,, even with four wheel disk brakes my emergency stopping distance would have been multiplied by infinity and taken to the depts of forever. Chances are the boat and trailer would be leading the car.
My point....3/4ton or heavier truck for heavy loads.