The 2.0 Vectra Turbo is basically a Vectra with a slightly detuned SAAB B207 engine.
The shopping list is:
The correct NGK plugs. PFR6T-10G is the SAAB specification. Don’t take equivalents from anyone else, you need NGKs. The car uses the plugs to monitor combustion and needs to have the ones it was designed for.
Decent oil. GM Dexos 2 or Mobil 1.
Check the thing is boosting right – bypass valve diaphragms can split, and on the high boost SAAB engines the intercooler can balloon a bit. From memory Evo 7 ones fit fairly easily if you want to upgrade.
Shell v-power, or 5l of gun wash thinners in the tank if you’re feeling tight. The engine management system monitors knock and can take advantage of high octane fuel.
Lastly, Noobtune can usually find an extra 40 or 50 horses for not much cash.
Beyond that the modifications get spendy – the downpipe and cat become limiting factors.