I wouldn't think there is options to buy just chapters, but it would be cool. It does seem that there may be enough new options to make a PHB and a DMG book. This may force 2 books with the DMG one selling less, so maybe that one in a PDF for me and players buying their own PHB option.
I was thinking of some lite options for free to get interest like the basic rules from Wizards, but think that through the playtest most of us will see enough of final or near-final things to decide before the final copy comes out.