[suicideneil]

They do come with a plastic diff cup yes- its a strong cup though, not the mushy plastic like traxxas uses.
Some brands/ versions do use the metal inserts, though it is still rare that the plastic cup fails as such.
I do believe that most if not all these 1/8 diffs have o-ring seals around the output shafts- yep, the ones Mike sells have little grooves cut into them for said o-rings; never had any oil leak out of my diffs (7k front & 5k rear).
The shafts Mike sells are designed primarily so that you can use traxxas drive cups or the stock sliders with drive yolks, but pretty much any 5mm bore drive cup/ yolk will fit just fine.