The Astra gsi is one of the least dated cars of its time its also a dream of a car to keep looking good. ( rust is not an issue with these motors )  
The one place to check, though, is the rear arches as they are usually the first area to go!.  
Stone chips and buggered fog lamps are next to check for and either is a sure sign of a high miler.  
As for color, the standard GSi is only found in red, dark blue, white and black. The most rare of the three, and probably the best looking (Yes, I am a biased), is the Diamond Black variety. 

Engine / Transmission 

The best thing about these motors is that 2.0 litre lump under the bonnet. The best ones to go for are the J or K reg models as these have the non-ecotec 150 BHP blocks.  
Although you could get lucky, as I did, and find yourself an early L reg with the ecotec head but the older bottom end, the result - a monster 150 BHP with better fuel economy.  

Both engines should see you through an easy 100K miles trouble free. Although you will need to keep your eyes on the cylinder head, you know, that milky oil and black water test!!.  

The transmission is said to be one of the downsides with owning performance vauxhalls, although in my experience as long as you tell it who's boss it will slip between gears no problem.  
As for durability the F20 gearbox belongs on a tank and you should have no problems as long as its looked after.  
I knackered my box by not noticing an oil leak and running the box dry (Well ,you live and learn!).  
The only problems to watch for are the worn bearings found on high mileage models. 

Electrics 

Again from personal experience, look for dodgy windows and mirrors. As for the ignition and engine electrics, try the simple diagnostic test outlined [here] to find any problems. (This might be a bit difficult in the forecourt, try to take it for a test drive by yourself and do it quickly round the corner!) 

Wheels / Tyres 

kick em! 

Brakes 

Major problems are warped disks, usually the fronts, and siezed calipers with the higher mileage motors. 

Suspension 

The standard shocks need to be replaced at 50k at the lastest, but this ain't a problem 'cause you were going to change them for sports shocks anyway, weren't ya!.