I'm going against most of the rest of the responders here. I think around $3,000 for a solid 1984 Porsche 944 is a good price, but you should expect to do more-or-less continual maintenance to keep it running properly. However, this is true of any 22-year-old car.
As for rust, Porsches after 1976 use galvanized sheet metal, so rust is almost never an issue. I had a 1978 Porsche 924 that had no rust whatsoever, and that was after driving it in upstate NY winters.
You will have lots of other repairs, and the 944 will require an expensive timing belt replacement every 30K miles. If you don't do it yourself, expect $500 for this replacement.
Nearly all parts are still available for the 944, so don't worry about that. If you enjoy the car and are willing to do most of the work yourself, this is a very satisfying car. It is peppy, handles great and has decent brakes. If you are willing to spend a little more money, look for a 1986 or later model. There was a major upgrade for the 1985.5 model, which included more power, better brakes, stronger ball joints and a better interior. However, it will probably cost you at least another $2,000 for one of these later models.
Finally, it's always great if you can find a Porsche that hasn't been beaten up on racetracks. Ask questions and see what answers you get, and look for signs that it might have been raced. These include racing seat belts, holes where a role bar might have been attached and painted-over numbers on the doors/bodywork.
I like older Porsches and there is a great community of owners out there to encourage you and help you solve problems. As long as you are willing to do the work to keep your Porsche running, you will find it educational and very satisfying. If you can't/won't do the work yourself, the car will almost certainly be too expensive.
I write professionally about Porsches and have worked on my own for many years.