Welcome!
The Vectra links are somewhat exaggerated. There are a few common parts, but it's in a similar way that VW, Skoda, Audi, Seat & Bentley are "the same".
The 9-3 used the floor pan of the Vauxhall, but they used a higher strength steel. The rear suspension is completely different, it's passive steering.
We have a very early petrol model. It's almost 12years old and it's fine. No rust. Everything still works. We have had it over 10 years with no serious problem. The only major repair being a new steering rack, probably worn out by the car spending most of its life being driven around locally in the urban environment with few long runs.
Check the front springs are not broken. They corrode and the bottom few inches snap off. Check the seat belts retract easily, they get "tired". Check all the electrics work, especially the windows and doors. That's about it.
The 2.2 diesel does have an odd fault. The rubber fuel lines by the injectors can go porous and leak air into the system. This results in poor starting, especially if the car is parked with the nose pointing uphill! Not too expensive to fix.
The electronic stability system works the rear brakes more than you would expect. Pads wear quickly. Sometimes the calliper on the rear seizes up and needs replacing.
If it all works now, it should be good.
