I changed from petrol to diesel cars years ago as the diesels have more torque. The petrols I had meant I had to use the gearbox a lot more but as the X3 has a 3 litre engine this may not be a problem. Running on LPG usually means a slight drop in power but again with a 3 litre engine that may not be a problem.
I have no idea what that Abbey weighs but I think the X3 weighs around 1750 kg so it should manage a caravan up to 1500 kg or so without any problem.
Have a look at the owners club forum, there will probably be owners with caravans that can offer practical advice.
Also, watch what they also require for towing, like additional transmission cooling and radiator fans. I looked at an X5 but back in 2005, then it was a load of plus plus, not just a very expensive hook.
If all that stacks up then you need to consider how much towing it will be asked to do; if that is but a few % then even a dreadful towing consumption really has very little but psychological impact.