Depends how you drive it (and also other factors as mentioned above). I can’t see why an old motor will be any worse on consumption that a more modern one if one chooses carefully.
I’ve never driven the V8 3-series but I did have a 113M 380 for a while and I pulled as well as your modern day 420 Scania 4-series, 420 FH12 etc, was above average on fuel (9’s) and most important to me, very comfortable and nice to drive.
I don’t think you’ll go far wrong. Take it for a run and let your instincts tell you if it feels right or not. Be aware that with it being a high power (nearly) top of the range motor there’s a realistic chance that it could have done quiet a stint of its life with the fuse out so I’d take the milo reading with a pinch of salt meself.
Good luck 