Also, I think I read somewhere that TNG would be very diffuclut and rather expensive to convert to Blu-ray. Does anyone know anything about this?
The answer to that is yes, it would be difficult.
The simple explanation - TNG, DS9 and Voyager were shot on film but transferred to video for editing. To create a HD master, every episode would have to be re-edited from the original negatives. Enterprise, on the other hand, could easily be released on Blu-Ray as it was created with HD in mind from day one.
Bear in mind that movies, let alone TV shows, from the 80's don't have the best reputation storing their original negatives properly. So, I wouldn't hold your breath for a Blu-Ray release.