From what I hear - the mini experimentals are the best producers (like Mini Nubians, for example).
Space is relative. Ours are more confined than we'd like, but since we're working on selling this property...we're not adding on. They'll get more room at the new place. I think you'll get lots of answers...but I'd say two would enjoy 40'x40' paddock with a small barn or a large dog house.
Goats really don't graze. We keep ours on a dry lot to help with cleaning up and parasite control. They get grain and hay daily.
Milk per day varies by individual. My NDs (and I don't milk to full capacity, I only start to milk once their kids have weaned which lessens production) give me about 8-12 oz per milking, so around a pint a day each.
YUP! They can be housebroken! Or you can just stick a diaper on them when they're in the house (easier on a doe).