can 280z door panels go on a 240z panel. I assume so but what do I have to buy in addition?

IMO, no, its not a bolt on. The 77-78 doors are completely different than 240, 260 and 75- 8/76 doors. The big difference is the striker and plate design. In the case of your question concerning interior panel fitment, I will tell you what I have learned:

Yesterday, I borrowed a 240Z door and I can not use it on my early 260Z as is because there is a big difference in the arm-rest pull handles attachment. The 260Z and up use a 1 piece unit whereas the 240Z are separate units. There are many differences in the stamping on the inner door panel steel - indentations, raised areas and welded nuts to receive the armrests and pull handles. Obvious outside difference in the door panel covering would be the location of the lock button. It was moved forward a couple of inches at some point - 77/78 for sure. I "think" the striker area of the panel might be different.

If you are willing to drill holes, weld on panels, pound out some areas, you might be OK with the outcome. But, I doubt it.

Here is link to hybridZ's discussion on door interchange. I now know that it is not completely correct!

http://forums.hybridz.org/showthread.php?p=1026341