The eSlick and the Sony Readers don't support DRM-free MOBI. The jetBook and Hanlin (BeBook) do. However, Calibre (has its own forum) will convert many formats to MOBI or ePub (or LRF) and this is enough to fully support the Sonys. I'm not sure about the eSlick, but I think it may eventually support eReader format which Calibre is about to add as an output format.
I would not choose plain text for reading because its formating is so variable. If you want text for Gutenberg, note that manybooks.net has them all reformatted into many formats and MobileRead has superior versions of many of them hand crafted by the members here.
Under low cost and good enough, I would put jetBook (not EInk but also not backlit and therefore not tiring on the eyes like standard LCDs) and the EZ Reader (the US brand of the Hanlin/BeBook) and perhaps the Sony PRS-505. The best deal on the EZ Reader is usually at Frys, which is currently out of stock for on-line shipping. I have three EInk devices, and the EZ Reader is the cheapest and also the one I use day in and day out. I recommend it highly. Note that I actually run OpenInkPot firmware on it with FBReader and CoolReader as alternative reading options. OpenInkPot is the best software out there for DRM-free reading, and it currently only runs on the Hanlin V3.