I built a raised garden with 5 foot high 4X4's. It is 6X20 feet. In the early spring when the ground is still frozen I used to cover it with heavy plastic. I still have the plastic I bought 9 years ago but I do not leave it up in the winter. Winter ages plastic fast. If you can monitor the heat build up throughout the day you do not need automatic vents but if you can not heat, from the sun could cook the plants. Been there done that! There is a "green house grade" of plastic that commercial growers use but it costs more. You can find a wealth of information on the net. Many sites have tips to reduce building cost.
Good luck with the project. If you enjoy gardening a green house can give you the opportunity to dabble at it year round.