Kim, that is a great set up and you certainly did get it for a steal. That wire shelf that you have, looks like you can pile a lot on it and it is sturdy? I want one across the end of my greenhouse, above the pond. To put plants on and hang light from. We looked at one at Home Depot over the weekend but DH didn't think it was sturdy enough. I'm so glad you posted that photo.
Bonnie, you could retrofit your shed. You can buy polycarbonate panels or use the sliding glass door. Jane's greenhouse is made out of sliding glass doors and windows left from her husband's jobs. The Harbor Frieght greenhouses are also very reasonable. They do need modifications though. Especially the big one. Still, the starting price is amazing. They are double walled polycarbonate.
Debbie, you would have to heat the water to keep tropicals alive. My greenhouse pond is sunk into the ground and insulated on all sides with styrofoam insulation sheets and I kept my greenhouse heated with an electric heater last year but the waterlilies all died. You are colder than I am. I did not heat the water. I will this year. I may use a horse trough heater or an aquarium heater I'm not sure. Yes, people use kerosene heaters in greenhouses. You need air vents.
Garden Web has a great greenhouse forum, many DIY tips there:
http://forums2.gardenweb.com/forums/strucs/