Putting aside some of the details of this and strictly speaking, shooting something that is 0.15 m tall and 500 meters away on a full-frame camera will require a nearly 100,000mm long lens (which doesn't exist, or at least isn't generally available). Two lines of text tall might be 0.3 m and needs a 53,000mm focal length (which doesn't exist). Ten lines of text (1.5 m tall) will require an 11,000mm long lens. 100 lines of text tall (15 m tall) will require a 1200mm focal length. Finally, we're down to a focal length that is available (not cheap or even affordable for most, but actually available). Useful: a lens focal length calculator.
The D800 has 36 megapixels, making an image 7360x4912 pixels. With 100 lines of text that means there's about 49 pixels of height per character. Viewed on screen at 100%, that's completely readable, perhaps 1-1/2 to 2 times larger than the text on this site appears for most. I don't understand the meaning of the "human readable (2.5mm height)" requirement and the specifics of that could negate the readability of this size, of course.
So, as I've calculated it, you'd need a D800, a 600mm lens, 2x teleconverter, and a beastly tripod to hold that. Shouldn't cost you more than US$15,000. You may want to figure a little extra in there to take some classes to learn to use such beastly gear, too. :)