I had the same questions, and here are my answers for now:
- for ebooks, Moon+ Reader Pro seems to approach the quality of Marvin from iOS most
- for PDF files, ezPDF Reader seems to approach the quality of GoodReader from iOS most
What I have most difficulties to find on Android: a "Dropbox plain-text editor", corresponding to Nebulous Notes on iOS. The closest seems to be Minutes, but it's not Dropbox-wide: it only syncs with a single Dropbox folder. I'm not sure Jota+ would be the answer, either.
Any advice on all three counts will be appreciated.