They were all imported from Germany.
Total production of all 400E/E420 was around 20,000-25,000 worldwide. It's an uncommon car, but not particularly rare.
Unless it's pristine, stock, and flawless the value is closer to what the previous owner was originally asking for it. The 400E (there is no E400) transmission is basically identical other than possibly a rear pump which was deleted as a cost-saving measure on later models.
I wouldn't add nitrous to an engine with 11:1 compression ratio unless you don't mind the risk of killing the motor. Want it to feel quicker? Swap in lower gears in place of the stock 2.24's, and/or swap in a 5.0L engine.
Make sure it's not actually low on antifreeze... coolant level should be at the tank seam (almost full) when cold. Only get a new OE/OEM Hella/VDO coolant level switch, the cheaper aftermarket ones are garbage. The air temp sensor doesn't do much but make sure it's plugged in, located at the air duct behind the passenger headlight.
In the video it sounds like the muffler fell off, might want to check that out too...