lets see. any E38 in general will likely have at some point navigation issues. like needing the unit replaced. various seals, like the intake manifold, these can also warp. water pumps are common. window regulators will likely go out at some point.
basically its all little stuff, its just not particularly cheap to repair. especially engine parts, sensors and other things, on the V12 compared to the V8.
although, i really would love to find one myself. they are real dream machines!
see if you can get ahold of service records or other paperwork. make sure you take it to a mechanic to get it checked out that specifically knows V12 bimmers, because there are little things that even i would likely miss if poking around.
do you have any pictures? how much are they asking anyway, and how many miles? what color? sounds interesting.
color: anthrecite metallic
wheels: style 89 E66, 19X9F 19X10R
ext. lighting: 8000k D2S & LED plate light
*want to get sport rear diff*