First, I have to say I'm jealous.
Regardless of the size of the structure and manufacturer that you settle on, you will need to check on the following before purchasing your structure.
Check your local zoning restrictions to see if they allow that large of a structure with out special permits.
Heating
You say that propane would be your most viable heating source. Check with local bulk suppliers regarding tank size, cost and permitting. There may be restrictions on how large a tank you can have and the cost of piping and heaters. Also check on propane pricing and delivery schedules.
Cooling
You will need some kind of cooling system for the warmer/hotter parts of the year. Even if the greenhouse has roof or sidewall panels that open for ventilation, you will most likely need fans for circulation.
Irrigation
Another cost to check on is a water supply. Will you use well water? If so, do you need to have a new well drilled. If you're planning on connecting to the municipal supply, what are their connection and monthly costs.
Once you have your start-up and recurring costs in hand, you may be able to convince her to start smaller. If all works well, you can always add on.
Best of luck! And again, I'm jealous.
