Zone 5a, huh? I was born and raised in MIchigan but I live in GA now. All my family is in Michigan though.
I got my greenhouse on eBay. I looked a long time. I don't think there is such a thing as not a nightmare to assemble. Okay, our's wasn't really bad, but not simple either.
Mine has doors at both ends, is 10x20 with six roof vents and Dutch doors. I love it!
http://www.ebay.com/itm/Harves...
I also have a Charlye's 8x10 at my son's. Better made greenhouse and easier to assemble, in my opinion. We bought that one used so we had to take it apart and reassemble. Still wasn't too bad.
http://www.charleysgreenhouse....