So, how much would it take to actually get this thing to be a reliable DD? BMW in a bubble since 97

https://www.ebay.de/itm/BMW-740i-E38-NEUWAGEN/283822959564

Is there anything mechanical on this car that wouldn't need attention?

I have no idea, likely a lot. Not being driven is pretty bad for a car. But I am intrigued as to why someone would do this...

I was hoping it was a Sport Package with the M-Parallel rims. This car was just a disappointment.

Properly stored cars last a lot better than most people think.  I would go through the fluids and filters,  turn the engine over by hand,  add a battery,  maybe a few other things,  and let her rip.  Then see what shakes out with a little running and driving.

I just bought an old Mercedes that sat in a garage for the last 15 years.  On that one,  step one is to go through the fuel system, Maybe the rest of the fluids,  then more than likely try and start it. 

I've had mixed results with low mileage time capsule Bimmers.  I had an E30 that was brilliant, but also had a 750iL that was a nightmare.  It seemed as though every bit of metal had corroded in the strangest ways.  Of course the radiator was toast ($400 mistake) and it had the typical bad cats $1400 mistake that I fixed with $200 worth of universal cats).  The copper in many of the wires turned to dust.  Seriously.  I would try to repair a wire with crimp connectors and they were corroded the whole way through, not just near the end.  I would try to cut it and strip it and the powdery fibers of the copper just fell off.

YMMV, but I wouldn't unless it was a real steal.

Time capsule Ford Ranger or W124?  Sure.  Bimmer or Rover?  Hard pass for me.