Bernadette,
I don't have a Signum, but as you may know they are very closely related to the current Vectra, and I have towed with one of these for the past 3 years. Firstly get a price for towbar fitting before you buy the car, I recall that the Vectra was a lot dearer than other similar vehicles because of the bulb failure warning circuit that I believe the Signum also has.
Towing has been fine, I pull a 1300kg max weight van with my 2.2 petrol, and it is dead stable, have to use the gears a bit on the hills but not excessively. Vauxhall have a max noseweight on the towball of 75kg and at that weight the van sits dead level on the back of the car. I believe the suspension is the same on both cars, so while the ride is a little firm solo, it is well up to the job of towing.
Good luck!
John