[Water leak in boot]

I would check window seal,you can buy a tube cheaply from your local accessory shop

[Astra GTC with 197k on the clock… worth buying??]

Not 197k ,not even if its a diesel 

[Change Oil Soon Light]

if your astra is the model with a tripometer button in the clocks ,put your foot on the brake and push the tripometer button at the same time turn on ignition ,wait 10 seconds and the numbers will begin to flash then 0000 .turn off ignition then back on ,should be reset. If not its done by accessing the menu on the left stalk