hub centric rings are exactly that. they are a ring that center your rim on the hub rather than centering off of your lug nuts. Your hub is 72.56 on an e53 and say for instance your rim inside diameter is 74(i don't know what the i.d. of the rep rim is) than you would need a hubcentric rim with a 72.56 i.d. and 74 o.d.
If you tighten it down with no ring installed it may not be perfectly centered on the hub and give you a nasty little vibration mutch like a tire being unbalanced or out of round.
no you don't need longer lugs with the rings, they don't mount between the wheel face and hub so the lugs length is just fine.