You named the best places to get a good used radio that I know.

Swapping to a New Gen flat pin radio is easy. You just need the parts listed below. I included the Sirius and Aux parts if you really get crazy with it.

Core Radio Parts:
65.12-6 988 275 New Gen Radio (current PN listed; oldest compatible PN for Sirius is 65.12-6 927 911)
61.12-6 913 957 Round Pin to Flat Pin Main Radio Harness Adapter
61.12-6 913 955 New Gen Radio Antenna Adapter
61.12-6 924 785 10-Pin to 12-Pin Radio Audio Input Adapter
61.12-0 140 718 CD Changer Audio Input Cable (if the car does not have 6-pin CDC plug)

Sirius Parts:
65.12-0 416 377 Sirius receiver module (comes new with 1yr subscription; older PN 65.12-0 398 140 is equivalent)
84.11-0 150 526 Sirius to CD Changer Adapter Cable
TERK SIR Mini Antenna (or BMW OE Sirius antenna parts appropriate to car)

Aux Input Part:
82.11-0 149 390 Aux Input Adapter Kit for New Gen Radio

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SOLD
2000 E38 750iL Highline Edition, Upgraded with the lastest 2006 X5 Electronics
MKIV Navigation, 16:9 Display, Bluetooth, TV/Video with Backup Camera, Sirius, and Aux Input