I don't use arks purely because we have a stable block in the same field as the pigs.
Yes they will use one end for the bathroom if its too large and they're shut in- my KK's use one corner of the stable when I don't let them out early enough
(two 65-70kg pigs per 10x8ft stable = far too large). You can 'toilet train' pigs somewhat and if they have outdoor access they will invariably go outside to do their business. It's actually easier to do 'toilet cleaning' duties if you can get them to all go in one area
You could always board off the back of an arc until you figure out exactly how much space they need (obviously increasing space if you get weaners)? Pigs like to cuddle up so if they have access outdoors 24/7 then they wont need much space in the ark at all.
Three pigs is great. I only keep boar (uncastrated) kunes now, but I've found boars and gilts to be equally friendly, they're a good beginner pig. Someone else may have had a different experience to me though, and be able to comment further here!
Getting some mangalitzas would be a good idea, because they're considered very rare now. If you plan to get a mix I'd get all gilts and introduce them at no more than 7 weeks if you're new to keeping pigs (unless you have a 30 acre field) or you may find yourself with hell on your hands (and a few savaged weaners).
Overall, if you're just starting with pigs I'd recommend the KKs over manglalitzas for handling and temperament (please remember to never turn your back on a pig - or trust it! ). However, if you're planning on eating them, remember you don't get much meat for your money on a KK and its reputably more fatty than other breeds. Good luck!