bruna - Welcome to the thread! X3
The Mobility Legs have an extra joint at the "ankle", whereas the default legs do not. You can see images of the Mobility Legs on their page under Parts. (here) The added joint can allow for a bit more versatility in posing. There has been some discussion on the advantages/disadvantages of the Mobility vs default legs in the thread, if you care to read back, but it's generally sounded like most folks like the Mobility Legs, though they aren't strictly necessary. (Edit: Of course, these sheep appear to have hooves rather than the standard limbs, so their version of Mobility Legs probably differ, but I would expect it's still a difference of the extra joint.)
Pictures of the "lady hand parts" are also found under the parts category. (here X3 ) The paw-like hands are the defaults.
I haven't got a Zuzu to compare to, so I'm not the best to answer that one, but from what I've seen of pictures of them together, they do look pretty similar in proportions.
As for how quickly they'll sell out, that's always pretty hard to predict, but I expect you would be fine to get your sleep. Carnival Oz & Storm were each limited to 30 and as I recall they were around for quite a while - a couple of weeks, at least? Possibly longer? I don't remember exactly, but it definitely wasn't a "stay up all night waiting" kind of rush.