Since one of your major expenses will be the polycarbonate panels, I would suggest doing some internet searches of suppliers. Also, just search previous threads on this forum for ideas. I am not clear about your question regarding the roofing material. If you mean that in zone 6 do you need at least twin-wall polycarbonate for the roof, the answer is that 65% of the heat loss is through the roof. Only 18% out the sides.
The height of the "roof line peak" should be as high as you can build and afford. The larger the volume of the greenhouse, the easier it is to cool in the summer. Roof vents- a minimum of 20% of the floor area. For your approx. 75 sq. ft. area, that would be 15 sq. ft of top, roof vent area. Heating- gas, whether natural or propane, is usually less expensive per BTU than electric. But, the plumbing, heaters, vents, etc. add to the cost of gas heating vs. electric. Fans- you need at least one complete air exchange per minute. If the vents- both top and bottom are not enough, adding fans will help bring up the air exchange to the neeed amount.
"best equipment to keep humidity in check" - For summer periods, the problem is usually low humidity. Misting systems work well to increase relative humidity levels. In the winter, high humidity is usually the problem. The only way to cost-effectively reduce winter relative humidity levels in the greenhouse is to ventilate and heat. The ventilation will bring the inside RH down to the outside level, and heating this air will automatically lower the RH.

To: Cusestaroble

Thanks for your help.

The more I research the more I realize how little I know about GH, hopefully I will not make any big costly mistakes. I did find other GH supplier on the web who is more reasonable, you just saved me quite a few bucks. :-)

I do have one question on the height of the roof line, you suggested it should be as high as I can build, the large the volume the easier it is to cool in the summer but on the contrary, isn't it will be colder in the winter? I usually start my perennial seeds in February, the snow season here is not quite over yet.

Thanks,
-Suzy-

In the winter, the upper areas of the GH will be warmer than the lower. But with adequate horizontal air flow mixing the air layers, the problem is reduced. If you are not planning on using the GH in the summer, then a lower roof line should be fine. However, if you are growing in the summer, it is often difficult to cool a greenhouse adequately, thus the reason for a higher roof.

FYI- Some of the suppliers can be very helpful when building your own GH, do not be afraid to call in and ask.
They can tell you placement and number of shutters size of heater etc.
Mike