Hi Greg
Not sure if you've seen this one:
http://obs.nineplanets.org/obs/obslist.html
A large and diverse range of observatories listed there. Some links may be a bit old and broken now.
If you want to get fancy, I have found that rendered polystyrene panels(exintex - R2.0) work well in the exposed walls. The panels are easy to work with. I know that many people will say to use metal sheet cladding on the walls relatively uninsulated so that you get quick cool down in the evening, but it can get very hot in there during the day if the walls are exposed to the sun and radiant heat. With the polystyrene and a white metal roof my observatory stays much cooler than the outside on hot days. I have a solar panel driving a 150mm 12V extractor fan in the side of the roof, which helps with heat or humidity build up inside.
I currently have a revised roll off roof installed/built by Bert (replacing the old 2 part roof) which has colourbond cladding, steel frame, bubble wrap insulation under the cladding and the solar fan plus a small whirlygig. The previous (split) roof I built was cumbersome, but had even more insulation in it - again counter to what some people will tell you to do. It had air flow ventilation (20mm gaps at the roof joins) and a solar fan. This setup was able to mantain an even 22-24 degrees on a hot 38-40 degree day. However, as you would realise the 'esky' approach of having no air flow ventilation is not advisable.
As far as dream observatories go, I really like the Russell Croman observatory megaconcrete pier, raised platform and clamshell on top - this seems to be a successful design, seen in many other high end robotic setups.
http://www.rc-astro.com/equipment/observatory/index.htm
guy