We plan to purchase a cable joint 4СТп-1-25/50 (Б) and a termination 4ПКТп(б)-1-25/50 (Б). Should we also purchase an earthing kit or there is a constant force spring, binding wire, and an earthing conductor? What is the use of a set КМПБ?

The mentioned joints and terminations do already have all the necessary items for a cable with armoring:
4СТп-1-25/50(Б) -earthing strap, 2 constant force springs, binding wire, tallow, solder:
4ПКТп(б)-1-25/50(Б)- terminated earthing strap, constant force spring.
КМПБ set is used for work with alum- or copper-armored cables.