You can clean up the residue you see, fill up to proper level, remove underskirt, and find out where your leak is. Jfoj might be right that it's the 0-ring...which, if leaking, is worth the 'expensive' IAT o-ring...but it could be leaking elsewhere. Clean things up, run car, and find source of leak.
Or, as you said, just replace everything. If you don't want to replace everything, you need to diagnose what's leaking...simple as that.
Check the entire system...I like using my 100 lumen flashlight for looking at stuff...you won't believe what a difference having proper light makes to clear diagnosis.
Upcoming NYC dates: Jan 8 and 27. See Facebook for other shows and PM me if you can make one...love to see you there!
Mangofy your E46!