Brian,
I'm with what the other guys said. However when I considered how I was gonna build mine, I did consider butchering a garden shed, however the more I thought about it the more the idea of building it from scratch appealed to me. Thats what I did in the end. You asked about timescale. It really only took me two weeks and I wasn't even off work. Just two weekends and the evenings in between. (I did it in summer).
The difference it makes to the hobby is immense. If there was one piece of kit I couldn't do without its my observatory. Like Jeff said, how many times have you spent setting up just for the clouds roll in. More importantly how many times have you not set up because you felt it might cloud in shortly and sometimes it doesn't happen. I can have my photographic setup up and running in less than 15 minutes and can be packed up and in bed in less than 10! With that knowledge you will then take advantage of the marginal nights you wouldn't otherwise consider.
I never got to complete the article on my observatory build. However some info is here;
webtreatz.com/content/view/43/1/
  