Hi Pingu,
There are several factors you need to consider in business planning and structural design of a greenhouse. I would classify into three and inform of questions
1. Concept.
Why do u want to construct the greenhouse?
Where are you setting up the greenhouse?
When do you start?
2. Customers
What type of plants to plant and to whom do you sell?
How much do they take and how much do they pay back?
3.
What is your financial situation or your start up capital? This includes the running cost and maintenance cost.
After considering this factors you make a business plan and start small and allow the business to grow.
Having this in mind you can now approach the different companies that offer packages in greenhouse construction considering their cost and durability.