Seen this, seems good value to me and they will give me PX on the Gallant. For swmbo so RWD/FWD makes no diference, seems a lot of car for the money.
http://cgi.ebay.co.uk/2004-Vauxhall-Signum-REDUCED_W0QQitemZ320332659936QQcmdZViewItemQQptZAutomobiles_UK?hash=item320332659936&_trksid=p3286.c0.m14&_trkparms=66%3A2%7C65%3A7%7C39%3A1%7C240%3A1318
To get back to the orginal question -
I have a 57 plate Signum Diesel 1.9CDTi which I think suits this vehicle as a workhorse and giving good economy with a good spec.
Not sure you are gaining anything with a big petrol engined version over an Omega or similar - you effectively get caught between 2 stools. The Omega is definitely a better drive, comfort etc. Signum more comfortable a ride than standard Vectra due to longer (estate type) wheelbase.
As regards faults - they suffer as does Vectra with a lot of broken springs. The diesels have the dual mass flywheel problem due to high torque which can cost a lot out of warranty (clutch plus dmf = £1500).
The Vectra C Forum is pretty good for info.
Bottom line - would not buy a petrol unless you do small miles and get it cheap.