The Bentley SC manual is relatively inexpensive, and although it has some errors (like pictures of some stupid Carerra parts), it may be all you'll ever need.
On the other hand, the complete Factory manual set is not only expensive, but sometimes kind of hard to navigate, at least until you get used to it. It is definitely intended for professionals, but I bought the set anyway.
The little Spec Book is out-of-print. Sometimes you can find them at swapmeets. The info they contain is especially useful if you are rebuilding.
Finally, if you can get a copy of the "Porsche Parts and Technical Reference Catalog-911 Models 1974-1989", you will likely use it more than any other manual. I rate it a "must have".