Welcome @RosieJoeSoapCo, yes, pretty much just lye and distilled water - milks will spoil if not used pretty quickly. I do add tussah silk to my MB lye and it is just fine, although I do have to stir or gently shake before using.
Master Batching is amazing...I MB my oils/butters and ready-to-use Lye Solution. But it's not something that I recommend for new soap makers...give yourself a good year to fully understand the soap making process and to know your recipe.
I have only MB'd Distilled Water and though it probably wouldn't hurt it any, I add my Sodium Lactate separately.
No, you cannot MB a milk Lye Solution. Setting aside that it is dairy and would spoil and it's not a good idea to store your Lye Solution below 65F (@DeeAnna can explain better than I can why), milk contains fat and fat + alkali = soap. So within a couple of hours of mixing your milk based Lye Solution, you would have a VERY dangerous container of soap.