I know that in UK, ferrets are often housed outside, but I don't know of anyone who does that in the US. Most of the ferrets you can find in US are produced on massive breeding farms (such as Marshall farms) and the genetics of the ferrets tends to suffer for it. I would be worried about a US ferret being able to tolerate the elements, and them getting sick and nobody noticing. They definately can't tolerate high temperatures and go into heat shock and die. I'm not sure how much cold they can tolerate-you'd have to ask someone who kept them outdoors. Ferrets are also great at hiding their illnesses and you have to keep an eye on them to be sure they aren't ill. Having them indoor/outdoor would probably be really good for them if you were always super careful about the weather. You would have to be careful about fleas and mites and stuff that they'd be able to pick up outside.
The biggest concern with outdoor enclosures for ferrets is escapage. They are escape artists and will dig/squirm/break the cage to get to freedom. The mini courtyard in your house sounds like a great place to let ferrets outside. It would have sturdy high walls that aren't very climbable and also prevent digging to freedom. You would have to put a mesh cover over the roof opening. Ferrets are about squirrel sized and thus the size of prey hawks and owls would be happy to snack on. The outdoor pen on ebay doesn't look terribly ferret proof to me. If you kept it on a patio or concrete area that was absolutely flat and weighted the top, maybe. On grass, they'd be out in a second. They'd just dig at the base of a wall until they could squirm out. Unless you have all sides including top AND bottom enclosed VERY sturdily, I wouldn't leave ferrets unattended in an outdoor pen. Once a ferret gets out to freedom, you can pretty much kiss it goodbye. Ferrets are the ultimate wanderers and it doesn't really occur to them to find their way back when there's so much cool stuff still ahead. Anything you make, please get a ferret shelter director of someone else similarly knowledgeable to look over your plans. They would be best able to pick out flaws and escape prone areas in your design. I found a pretty awesome looking ferret/cat enclosure on a UK website:
http://www.lazy-ferret.co.uk/cages/cages%20(6).htmNote the dig-proof concrete floor and all sides enclosure. Flaws I see with that design is the height of the structure and the second floor area. Ferrets are great climbers in that they love to climb to great heights, but they are not great at holding on or getting down. Some will drop down and a fall from the wall of that enclosure could kill. The second floor is dangerous for the same reason. Ferrets wrestling around would fall right off the ledge and get hurt. I wouldn't make any ledges or climbable surface higher than 2 feet. Place some plywood or plexiglass to stop climbing past that point. I'd also make a full second floor or box it off with walls and a ceiling so the ferrets couldn't fall. The dryer tube tunnels going to the second floor is a great idea to minimize falls when going up. You'd also want a bunch of nest boxes for sleeping and staying warm.
As for boys/girls, most ferrets you'll find in a pet store are already spay/neutered and descented, even the babies. They don't pee to mark, but they also don't litter train the way cats do. They pee/poo in corners generally, and will often use a litterbox if one happens to be in the corner they chose to go in. I have a specific area my ferret is let out in that is plastered with litter boxes because they in any convenient corner when they have the urge. Some ferrets are better with always visiting the same corners, but it shouldn't be counted on, even if they're good most of the time. If you're not comfortable cleaning ferret messes off the floor all the time, they probably aren't the pet for you. This happens with both boys and girls. The only real difference between genders is the tendency in boys to be cuddlier, though that's not always the case. Baby boys are crazy and wiggly and my older girl is very cuddly. Pairs or trios is a good idea. They do best in groups, mine is single by necessity because she beats the stuffing out of any ferret she meets and it would be mean to make another ferret put up with her super-dominant behavior.
