Pads on elbows are essential, slide shorts and knee pads imo are less important.
Elbow pads will enable you to land on your elbows when diving much easier, they will stop grases to the elbow, lower arm and palm of the hand on some makes of pads. (Eclipse elbow pads i have found are the best for this)
Knee pads and slide shorts are less necessary imo, with the latter being slightly more important, you should never put that much pressure on your knees that you will need extra padding that a decent pair of pants will not give you.
It does depend on how old you are and how the ol' joints are holding up. I am 17, have never had any joint problems so only wear elbow pads. However if you are older have a dodgey knee, or think you are putting too much pressure on your knees, then knee pads are probably best.