I know for fact (have one in mine) that the C43 and CD43 are plug-n-play in 92 or newer. Might work in a 91. 90 & older will require some diddling with the pin assignments on the back of the head unit (search archives for George Mann's conversion).
If the Dennison iPod adapter plugs into the CD changer cable in the trunk, then a newer e39 CD cable will have to be strung also, and I think there is one more caveat that requires another wire to either be powered in the trunk or run back to the head unit. Good news: there is another way around this. PAC/iPac makes the iPac-BMW adapter similar to the Dennison and it plugs directly into the back of the c43 or CD43 head unit without messing with a new CD cable to the trunk. I'm not sure if it displays the iPod artist/title in the RDS display. The PAC adapter sells for approx. $110 on ebay.
Either way, one must forfeit/abandon the existing CD changer in the trunk. That can also be made to work with an Alpine CDA-630 or CDA-634 changer with the new e39 cable and extra wire. For that setup, there are iPod adapters on Logjam Electronics that preserve the CD stacker function in the trunk and also allow an iPod.