Had once 520i -74 (kf), sunroof, no a/c so can’t help with that.
Iirc blower motor shrouds etc needs to be air tight (and everyting downstream for that matter). There might be different o.d. blower motors (two ithink) so be sure to get right one. If it feels like a cows breath it might be time to get new?
See what w&n offers for E12.