There will be loads of more informed people than me but here are some pointers....
These are the issues that I had or avoided....
Check all dash lights come on and then off when turning key. Should be airbag light, Engine Management light x 2, Traction Control light. oil light and handbrake light.
Check rear suspension coils, these tend to fail at 90-100k. If snapped, £120 to fix.
Same with front wishbones - make sure no knocking noise from front suspension. £100 or so to put right.
Proof of cambelt change at 40K is a must. Water pump at the 80k cambelt change is ideal too.
Rear pads usually go at 90-100k miles also. You get no warning, just grinding and then discs need doing too!
Unless this is ex-police, it should have satnav stereo. Sometimes, these get removed. These cost over £1,200 at Vauxhall and are rare to find on ebay, etc.... Colour screens are an extra that people rarely took up, so if this has one, expect to pay £200-300 extra.
Rear blinds were removed at around this age so there may be one blank button on the control panel (bottom left).
Make sure you have the passport with all stereo codes, etc...
Keith