Then you will want to convert to .azw3 (KF8) not to .mobi (KF7) since .azw3 is a more recent version of Kindle Format which is fully compatible with anything EPUB can possibly handle, and plays nicely with cover metadata, and other assorted treats. (It
will still read the legacy format.)
As far as which reader to get, that might require it's own thread.
Personally, I suggest the PW2. The Nook Glowlight has locked off the majority of the memory so only downloads from the Nook Store can be read; only a small fraction is available for sideloaded books. Of course, that's also when they removed the MicroSD slot.

The old SimpleTouch didn't do this IIRC, but it doesn't have a light.