Both the 8 & 16v Astras arent too bad on fuel when driven kinda normally,but both can be a bit thirsty if thrashed.
Key points to look out for on the mk2 Astra are:
Rear wheel arches-mud collects inside the inner lip and rusts rapidly.Infact i'll be suprised if you can find an Astra
without rotted out arches! Spray liberally with 'Wax oil' if you find a good one...
CV joints-Get the car in a wide open place and drive it on full lock both ways.Listen out for knocking.
Fuel leaks-Often the fuel hoses weep petrol which fills the cabin with vapour.
Water pumps-Get yourself under the car and see if theres any coolant leaking.
Power steering-check pump for leaks and also check fluid level is at correct height on a warm engine.
Other than that-they're pretty easy to work on and parts arent too expensive.Same cant be said for insurance...
Always trust the monkey