Be forewarned Dave,
The Harbor Freight greenhouses are great;... yet to keep them from major wind damage in the future, do need internal supports.
I love mine and have been using it 3 years now, yet have heard horror stories from others who did not build a bracing system inside before getting to involved in plants and other developments.
All the posts inside mine are of course the typical 2 feet deep and the cross beams create hanging plant and shelving but more importantly are installed with hurricane strapping. Pilot holes were drilled into the aluminum framing and screws were added through it and into the post framing before the panels were added. These are great greenhouses! The instructions are not difficult as some reports have said, and my sliding doors work perfectly after 3 years.The polycarbonate panels can be extended in life with exterior shade cloth that can be removed in Winter and rehooked in the Summer. They will get very hot inside in Summer without it.
A fan added to the back panel will help with the 4 vents open.
They work great in the Winter in our more Southern areas with the addition of a small oscillating tower type heater or one of your choice. These are best suited to keeping plants alive in the South and likely would not help much in the North, and especially without supports as the top likely would not handle the weight of heavy snow.
This isn't a great pic but its to show that basic window fans (and very affordable compared to 'greenhouse' fans which are rediculously expensive) work fine. This one fit in here, is a twin fan unit meant to fit house windows and can either exhaust or blow or a combination. It even has a thermostat and under 40 bucks!
Happy Gardening!
David