Brock,
Value for car as described is probably fairly low.
Pics would help. Get a snapshot of the VIN plate near the hood latch if possible.
Yes, the official MY designations started as 1978.
But the production began with a set of production prototypes in very early 1977, several of which have been found in general circulation recently, so there are a few very early cars out there, waiting to be rediscovered.
The story gets painted pretty well within these threads and varous sublinks.
Early Porsche 928 restoration - "Number One", Preproduction Press Car (#1RC) - Rennlist Discussion Forums
'77 ('78) Porsche 928, "Number Six", Preproduction Press Car (Updated 3-18-13) - Rennlist Discussion Forums