Answer: To be honest, the plants you named can usually be kept in the house over winter with no problem: amaryllis are fine on a window sill, as are geraniums and hanging baby's breath (they may get leggy but they will survive), and seedlings can be started under lights and then moved to a coldframe or other sheltered spot for hardening off.
Home greenhouses are always an adventure and it takes a lot of planning (and usually some experimentation and adjustments) in order for things to come out right in the end. You might find the following article about Home Greenhouses http://www.wvu.edu/~agexten/hortcult/greenhou/building.htm very helpful in deciding if you really want to try to do this and if so, which type and what materials would be best for your situation. A greenhouse is normally quite expensive to heat no matter what materials you use, so take a careful look at the heating requirements section.
Good luck with your project!
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