Slider or not depends a lot on the 5er, and how careful you are. If the 5er has squared off front lower corners you may well be advised to get a slider. But if it has rounded off contoured lower front corners you may not.
A good friend had a 5er with square front corners and short bed and knocked out his rear window on a tight back in. Later traded it for a newer 5er with contoured lower front corners and had no more problems. Although part of that was probably him being a little more careful.
An air ride pin box is not necessary. BUT. It does dampen the shock between the truck and trailer and makes for an overall smoother tow.