Welcome to the board, lots of good people here.
1st thing you'll want to bone up on is what we call "fender tags".
This car started life as dark green with green interior (see 2nd row from bottom)
N in VIN and E63 on tag (bottom left) is 383 4bbl.
As an investment, color-changes are generally regarded as a demerit, though there are also folks who don't mind and some who dislike green.
Non-original modifications are also widely considered non-optimal, though the auction market right now likes restomods.
Gran Coupe convertibles are pretty rare, as are 4-speeds.
To me, quality and condition are more important than bolt-on stuff like the headers & ignition, which are fairly easily "corrected" if you want.
The trans has been swapped, Shaker installed. #s matching & restored-to-tag is a big deal at the sharp end of the market.
Ultimately, you have to decide for yourself what you want in a car at a price where you won't get hurt down the road.
If you really like the car as it is, and it checks out, there's value in just being happy.
Being that the car is at a dealership, the actual owner likely values the car at 10-20% less than $69,900.