I did use Nirvana, and it has a good, reliable tickler system (called Scheduled). Now I use Doit, and it has that exact same kind of thing (also called Scheduled).
I don't think either of them can manage attachments, though; so you'd have to paste a link to Dropbox, GDrive; Evernote etc in the task's comments if you need to keep scanned documents "integrated", or take a look at Zendone or IQTell for more elaborate Evernote integration.