Probably the rear arches, sills, top of windscreen. Give it a good inspection with a big torch underneath. You can wire brush/use a cup brush in a drill and underseal afterwards (hammerite underbody seal with added waxoyl is good) £5 for a decently sized tin.
Hello i have found some informations for the astra mk3
for rust and i have 3 photos showing the places of the car that you should check for rust
the problem is i dont know how to send you the pictures in the text
thanks 4 advice, i'll check it out carefully, but it'd be nice to know what r the problem areas - rear arches sound common with vauxhalls. Ive not seen many rusty mk3s around tho so fingers crossed. Its a 92 car im gonna look at.