Slider shorts will increase your endurance as your hips will get a constant bashing if your going for hours on end throwing yourself in there.
I have always found that knee pads are always a must, no matter where on the field you play, imagine kneeling onto a peg when your behind a bunker, if its a metal one you will be in excruciating pain. True story. You only get know pair of knees and you want to be on your feet for a good while longer, don't skimp on cheap protection get something good
Arm pads are there as Bolter said, to keep your skin on, they also increase the likely chances of bounces when your shot in the arm (very handy I have found).
If your arm pads or jersey doesn't have any palm protection then I would suggest getting some gloves, because when you slide on, you will always slide with one arm and stop yourself from sliding too far with the other, its that hand/palm area that might get scratched or uncomfortable after a period of time. I once played the snake for an entire weekend at bricket wood when it was astro turf on very hard ground last year and both hands got blistered to hell, a good 1inch diameter patch of blistered skin on each palm... I can say that going into work the day after with these two very tender areas on my hand got me a lot of jokes
but couldn't play properly for about two weeks. I have always worn protection over my palms since, no matter what the weather is!
Conclusion - you only get one chance with your body and bones, don't make yourself regret not wearing protection in the future when your knees are a bit 'dodgy'
Welcome to our sport!