for what it's worth, I think it's always good to rationally weigh up the various options. You're probably going to change the car anyhow. If you opted for another astra, all of the risks you face with an alfa will be present with the new astra. When you buy secondhand cars, its all about how well they've been cared for - generally speaking, they all left the factory working. So you might be a standard Astra and find that the timing belt hasn't been done (or done properly or whatever).
Yes, parts might be more expensive for Alfas, but in my experience, the difference isn't huge. There are maybe fewer specialists around, but you can generally find a decent mechanic who will do a conscientious job. You can download the factory manual for the ph1 cars and hand the mechanic the relevant sections if you want him to do a big job.
The 916 series (I have a ph 1 spider) is really well manufactured and put together. I've been really impressed by the quality of materials used.
the only real rust issue you're going to have (assuming the car hasn't been driven around beaches or whatever) is the floorpan, which won't have been galvanised. that said, most other cars don't rust in the floorpan, normally - you'd be unlucky to find a Toyota or bmw with a rusty floor (sills, yes! ) But the dealer will probably be able to get the car up on a ramp for you to have a good look around. Rust will be fairly obvious, and you can check the exhaust at the same time, which is probably the most likely part to fail through corrosion.
On these forums we naturally read about all the things that are going wrong and so its understandable that one might thing these are constantly failing. But for most people, the cars work normally all the time, and are actually quite reliable. Any second hand car is an unknown proposition, until you get your hands on it and get it serviced etc. but Alfas don't really bring any more risk than more run of the mill cars.