Hi, it's been a while since I've been here but figured this would be the best place to post this query.

I'm interested in lunar photography, and a book I have on the subject of astrophotography has pointed me in the direction of either a 500 or 1000mm Reflex-Nikkor. My budget has me leaning toward the 500mm... as well as its usefulness outside the realm of astro/lunar photography. I primarly shoot landscapes, nature, and old buildings / faded advertisements.
What are the pros/cons of these lenses? Any particular ones to avoid?

My current shooter is a Nikon FG, and my very limited lens selection for it includes the Micro-Nikkor 55mm, Nikkor 24mm wide, and Zoom Nikkor 80-200mm. I don't have a teleconverter etc.

Thanks for your time and any help you can provide.