All the EInk devices sold today are using the same screen technology. For reading plain text, the font also effects the apparent screen contrast but most devices give you some control over fonts. Probably any of the popular EInk devices would suit your needs, and you could start with the
E-book Reader Matrix.
Note that Sony appears to be preparing to introduce new models, and practically everyone else (except Amazon) is in the middle of switching from MOBI to Adobe Digital Editions (PDF and ePub) for DRMed ebooks.
Calibre can format shift your ebooks to the optimal format for your device. One advantage of ePub is that you can include your best font in the ebook. Calibre does not yet support adding enbedded fonts to ePubs though.