How much power would you like from the 7? The N63 reverse-flow twin turbo V8 has been through a few iterations, and for the model years you're looking at, it'll be the improved Tu2 model that addressed a number of issues from the first-gen N63 hot-vee. Oil consumption, brittle rubber and plastic turbocharger oil and coolant lines and fueling-related misfires are the top concerns with the V8. Like all their V8s, BMW's recommended 10K synthetic oil change intervals should be addressed much sooner, perhaps around 6,000-8,000 miles per change.
If Mrs. Chief doesn't mind road trips with 120hp/100ft-# less output, the inline-6 turbo B48 went into the 740i and most of BMW's U.S. fleet at the same time. It's a reliable, fairly proven engine connected to the same ZF8HP transmission.
If you don't mind the differences in tech, you should take a look at a 2015-up Audi A8 as well. It has the same 4.0tt V8 from the S6/S7 mated to the same ZF8 auto and fixed AWD with Continental-supplied air suspension. BMW's iDrive infotainment is far more advanced than the MIB MMI in the Audi (that didn't even get Carplay in the D4 generation), but the A8 offers a ton of room, comfort, additional aluminum construction and value, now that the last somewhat-analog D4 generation was replaced by the new D5 model.