As King mentions, the MkIII is much more prone to damage by improperly disconnecting the power source then the MkIV. I've disconnected my MkIV dozens of times without disconnecting the battery with no ill effects. Just let it go into "quite" mode (no i-bus traffic for about 15 minutes) before disconnecting it.
I would try to both test your MkIV in his M3 and also test his MkIV in your X5
