I would keep the car. In my opinion, a first year Euro speciman is valuable, even if it is an automatic. I would restore it over time too. Bring it back. If you can do some or all of the work yourself, then you will save considerable money. Learn how if you don't know (look at the Rennlist site for lots of info on resto and repair for the 928). It won't be cheap or easy, but very doable. These early cars are much easier to work on than the later years. They are also lighter, and drive differently - more like a sports car. I've had several years, and my current 78 is my favorite. It was the first year, and they do not make them anymore.
You can pick pick up a driver for a few thousand, just to get to know the brand. Store that 78 (read a few threads about proper storage techniques), drive a later model for awhile, then start to gather parts for your project. It will be more difficult to find parts for the 78 however, just take your time with it. I think you have something special, if done right, over the long haul.
PS - don't just fire it up after it has been sitting for so long. Pull the battery strap, drain the fuel tank of the old fuel, clean all grounds on the car, especially the ones underneath and at the battery, pull the plugs and spray some oil down the holes. These simple steps can keep you from damaging a dormant engine.
