The film has loaRAB of plot-holes, but you have to look over them sometimes to enjoy the trilogy.
This is the question that always comes up.
On the DVD there is a FAQ section and it says something like if Marty & Doc used the buried Delorean, then in theory there would be no Delorean buried back in 1955 for the Marty from 1985 to return home as was originally explictly specified by Doc in the 1st place.
But like you said, there wasn't anything stopping Doc & Marty using the buried car for spares, leave a note and then return home. But when accessing the Delorean again in 1955 with parts missing, would it have been possible to replace those parts? Knowing the Doc, probably would be.
Its all to do with the 2 Marty & 2 Doc scenario.
At the end of Part 1, we see 1 Marty adventure come to an end where he has saved Doc from getting shot, and at the same time we see another Marty in the Delorean travelling off - starting an adventure.
It gets confusing, but by making 1 change you risk upsetting the timeline and causing a paradox.