Shopping for a Greenhouse Kit

I quickly learned that there are many purveyors of greenhouse kits that can be shipped in boxes and assembled in one's backyard. Frames can be made of wood, resin/plastic, or aluminum. Various shapes are available, including A-frames, barn-type roofs, onion shapes; stand-alone and leanto models. Bases can be fashioned by the homeowner from treated wood or can be bought as fitted galvanized steel bases provided by the same greenhouse manufacturer. Coverings can be polyethylene film, single- or double-layered panes of solid polycarbonate, or even glass. The double-layered polycarbonate is constructed like corrugated cardboard and insulates better than single layers. The polycarbonate panes can be of varying thicknesses. Single-layered polycarbonate panes can be clearer to see through, but don't insulate as well. A clearer type of double-layered polycarbonate may allow one to see clearer through it, but can allow hot spots on leaves because it doesn't diffuse light as well. The thicker polycarbonate may be more expensive, but can save on heating costs. A larger greenhouse can provide more space for growing plants, but can cost more to heat.

In short, there are lots of trade-off decisions to make. The more you can afford to spend; the greater the number of trade-offs. Here's what I decided so far:

o I will purchase both a greenhouse and a hydroponic system. The hydroponic system will come with a lighting system that will let me get my seeds started in the basement; starting the seeds in the conventional manner though, not through hydroponics. After I have more experience with hydroponics, I can consider starting seeds using the hydroponics.
o I want a greenhouse with dimensions approximating 9' x 12'.
o I want glazing with at least 8mm thick double-layer polycarbonate, to lower heating costs, as compared to the 4mm or 6mm glazing I saw on some kits.
o I will buy the manufacturer's steel base, rather than try to build my own from wood.
o I will investigate using a greenhouse heater that uses liquid propane (LP), rather than electric power, because I read that LP heaters might be less costly to operate. If that is the case, I would consider extending a gas line from my existing submerged LP tank that I already have installed to power my home's emergency backup electric power system. (I need to check on how venting is accomplished when you heat a greenhouse with gas.)
o Also, I will try to ensure that whatever electric power is supplied to the greenhouse and to my hydroponics system will be connected to the emergency power system, to save my plants if there is an electric power outage.

I am about to choose now between the following models:
o Juliana Premium 9.9 (at http://www.greenhouseoutlet.com/premium.shtml) that I can get for $3,099, including free greenhouse base, electric greenhouse heater, and shipping. This has 10mm double-layer polycarbonate with a mill finish aluminum frame.
o Nexus Cultivator Premier (at http://www.mynewgreenhousepurchase.com/) that I can get for $2,675. It has only 8mm double-layered polycarbonate, but includes a white-painted aluminum frame that is an expensive option for the Juliana greenhouse. Shipping and steel base are included at this price, but I would have to buy a heater separately (which would support my going for a gas heater, if that proves practical and desirable). The white painted aluminum is said to make it easier to keep the greenhouse cool when outside temperatures rise above 95 degrees F.
o Riga IV s (at http://www.outdora.com/13575riga4s.html) that I can get for $3,098.35, including a steel base, with free shipping. This has 8mm double-layered polycarbonate on the sides, and 10mm double-layered polycarbonate on the ends. Unique to its onion-shaped design is a dutch door and a big rear window that can allow cooling during the summer, while discouraging our dogs from entering when the lower portion of the dutch door is closed.

I am leaning towards the Riga IV s greenhouse, even though it is not the least expensive, because it is the only one with a dutch door that will make it easier to keep the greenhouse cool, while living with our dogs. This deal also permits the freedom to choose a gas heater if it is found desirable to do so. I plan to put the order in for the Riga greenhouse this week if nothing else arises.