1. They can be serviced by any half decent mechanic. Even I can do the basic service myself, and if I can do that then anyone can
2. Well depends on the mechanic and how much they want to con you out of your money. I would not think it would cost any more than any other 6 cyl wagon?
3. Depends on the car, something went majorly wrong with my car that could not be forseen, (a suspension part failure), and cost a lot to be repaired, thank god for insurance.
4. I had my R33 skyline for 2.5 years, and in that time I did my best to destroy it, I didnt know what mechanical sympathy was. I flogged the crap out of it and it took it all. My opinion is that Skylines/Stagea's are well over enginnered and can take a pounding, and if your parents wont be doing any mods, then they shouldnt have a problem if they get one in good condition. Again with any type of used car there is a possibility that they get a lemon, that is just the way it is. So try to get it inspected thoroughly by eithr RACQ or a trusted mechanic.
Oh and tell them to wave to other stagea owners and sign up here