I'm not normally a fan of grey leather, But I like that... Suites the E39.

If you were going to change to black, Do it properly... Dash/Carpet/Seatbelts/Doorcards and seats... It would look odd with black leather and light doorcards... And if you change the doorcards it won't match the dash.

I presume there similar to an E46, If so it's not that difficult to remove the interior.... Hardest part would be the carpet around the heater matrix.