Ok, so, onward and upward,
A few notes about situation. I keep saying a greenhouse is in some ways like a shop, the bigger the better. But it's also true that the structure should be as simple as possible. That's why I've never been fond of those automatic vent openers. If you depend on them, and something unforeseen goes wrong, and they don't open, you can literally lose an entire greenhouse worth of plants in one day, which means losing a whole season.
You really will be happiest if you keep it as simple as possible. I've known water gardeners who go for every kind of gadget, and can discuss with you the nitrogen cycle in excruciating detail, along with a cure for every little step. These people invariably give up on their ponds. Others just have a pump and a rock filter, and enjoy their ponds for years.
Sealing ridge vents in a double walled greenhouse is a major problem. If anyone can solve that problem, Marty can, but why introduce the complexity? You want a greenhouse whose total heating cost in winter is one box of candles. All the candle does is to get the air moving, so there are no cold spots of dead air. You're actually relying on stored heat in the ground, and in the water, as well as the warmth of the ground underneath the pond. There's plenty of heat there, even for tropicals, as long as you have a closed system. Get too fancy with vents and things, and pretty soon you're into the world of heaters, and moisture control devices, and you find yourself a specialist in climate control, instead of being a gardener...
Now, the big question: north/south or east/west. Which way does the ridge line run in a free standing greenhouse? Most people will tell you to run it east/west. Hard to argue with that, the long side gets the most direct light, you can make the north wall solid, since the sun never shines that way, etc. I've done it both ways, and I'm here to tell you I actually prefer north/south. If you do it east/west, every single one of your seedlings will be a 'leaner'. Not by much, but sometimes more than a little. Not only that, but a greenhouse isn't really a microclimate... it's a bunch of little microclimates, and that's a good thing. It's actually easier to manipulate them in a north/south greenhouse.
Ok, so utilities. You need water and power in there. 12/2 w/ground is all I've ever run to a greenhouse, and it's plenty for lights, pump, fans, and music. Water's trickier. Standard buried PVC is fine, but make sure it's buried at least a foot all the way to the greenhouse (two feet is better). This will keep the water cool in summer and warm in winter. But once it gets in there, you have a problem. Plants, especially seedlings, will go into temperature shock, and the greatest cause of that is watering that's too hot or too cold. That's why you need some kind of reservoir. I prefer a large (50+ gallon) plastic trash can with thick walls, so I can attach a garden faucet at the base. You want the hose to go into the top of that, and you want to keep the cover on (just to make it harder for the mosquitoes.) Keep it full, and use that water for your plants. The bottom of it should be at the height of the benches, or a little higher. It's useful to put a *very* light dose of something like miracle grow in there, since you'll be using a soilless mix for all your plants.
Before I give up for the day, just a word about actual use. I think I've said before that southern gardening is not like northern gardening. Here in the frozen north, what I miss the most is three season gardening. Here, we only have one season. Down there, you have spring into very early summer. Then you have a dead time in midsummer, when being out in the garden after say 9 am is an activity only for the most dedicated. Then, in early august, it's time to start seeds for the summer/fall season, when you can get an entire second crop. Then, once the frost hits, you can have an entire growing season inside the greenhouse. I miss that. A lot. Here, all I can do is mulch the banana trees, and hope they'll come back (they did this year). There, I could start the bananas form seed in winter, grow them outside, dig them up and plant the stalks in the ground in the greenhouse, and then when spring comes plant them back outside, and get fruit on them. There are lots of semitropical options the greenhouse will give you. You mentioned boston ferns. Well, pick your favorite, with the growing habit you like best. Build a two foot by two foot by 4 inch box, fill it with soilless mix. Now, take that fern you like, give it a hair cut, cut the root ball into one inch squares, and plant those in the box. You will never have to buy another boston fern again... when you need one, just cut a four inch chunk out of there, pot it up, and refill the hole with potting mix...
Ok, I think that's enough for one day... as you can tell, I could go on and on...
Thanks,
Bill