Whether or not it would fit, and it seems it won't, it is not a good idea to swop towbars. It is required to do a hefty job and I would feel the peace of mind to get a new one would be worth it.
On the difference between the petrol or diesel models it is not as clear cut as has been suggested. Yes, the diesel is a better tow car but you have to decide how much towing you will actually do. The diesel models are usually more expensive and often have higher mileages compared to the equivalent petrol. There has also been some issues with injectors on early D5 models.
I have a petrol S80 and can say that yes, I use more fuel when towing but over a year it amounts to about