2) Type M "cement" commonly found at Lowes and Home Depot, is actually a mixture of 3 parts Portland cement and 1 part lime... adding this to sand at the ratio of 1/3 (type M "cement" to sand) will yield the equivalent of type M mud.
3) You have three options: Mixing from scratch as in #1 above... using a semi-premixed type M "cement" and adding your own sand as in #2 above... or purchasing a total premix, such as Quickrete (also found at Lowe's, 7-Eleven, et. al.), that has all the necessary ingredients, "Just add water!"
The quality of the end product is proportional to the amount of convienience you are willing to sacrifice.