These cars are now 20+ years old and most have had several owners. Some were tracked hard, some sat for years, and not every previous repair was done properly. None of the items below are expensive, but several of them can take the car off the road in a way you really don't want, so it's worth going through the list early.
All models
Hub carrier bolts. These are a known failure point. When they snap, the suspension at that corner collapses, often at low speed and with no warning. Inspect them, and if there's no record of replacement, renew them.
Rear toe links (tie rod ends). These are cheap, and their failure is just as serious. Check for play and torn boots, and renew them if in doubt.
Brake servo vacuum hose and non-return valve. The hose splits, frays at the ends or collapses, and you lose most of your brake assist. Vauxhall replaced the original hose with a revised one (red stripe) under a campaign. Confirm yours has been done, and look at its condition anyway.
Springs and spring top caps. Spring retaining caps, and the springs themselves, can corrode and crumble. This applies to OEM parts as well as aftermarket coilovers. Spend five minutes feeling each one: they should be solid, not flaky.
Tyre age. A low-mileage car can still be sitting on old, perished tyres. Check the DOT date codes, not just the tread depth.
Water ingress. Cars kept outside can get water into the cabin and electrics, which causes intermittent faults later. Check carpets and footwells for damp.
Normally aspirated and supercharged (Z22SE)
Timing chain. The Z22SE chain, especially on early engines, can wear, and a failure is very expensive. If you hear rattling or other unusual noises from the chain end of the engine, investigate before it gets worse.
Rear wheels: look for the "HT" stamp. On the standard 5-spoke alloys, early rear wheels were recalled because they were prone to failure. Replacements have HT stamped on the inside of a spoke. If yours don't, they need replacing.
Turbo
Fuel hard-pipe clips at the coolant tank bracket. The plastic clips holding the two fuel pipes go brittle and break, and the pipe can then work out of its connector and spray fuel in the engine bay. It's a one-minute visual check and a cheap fix.