The greyscales are only interesting in case you want to display images in the device (essentially, if you want to read comics). In any other case you're completely happy with 2 greyscales, black and white
A 5 or 6 inch screen makes no sensible difference if you read files which are adapted at the screen size. However, many, many PDF's arent. In case that they aren't adapted, you'll have to go for the bigger thing, that is, 6 inches. Still, if the file is a research paper or something which really needs a DINA4 paper sheet, 11 inches, you'll sweat quite a bit to read it in a 6'' device.
Astak Mentor supports all the big formats so that you don't have to go around converting things, aside of a slot for an SD card if you need more storage space. However, it is much less agreable to handle than Cybook Opus. Nevertheless, if your wife is going to read mainly PDF's, you can be discarding the Opus.
BTW, you can go to the Flea Market and scout around. There are people who really have profitted from it, in the sense of acquiring lots of incredibly good things at a great price.