Sounds like the seller is being pretty honest. He really could have said "great condition" and not taken pics of the missing trim. He really didn't need to post a video of the ticking engine but he did.
Having said that, I think it's more than noisy injectors. My S430 is making the same noise (but much quieter) and I have heard other W220's doing it. It is definitely an upper-end engine problem.
If you have rebuilt a Jetta engine, you have the knowledge and tools to fix something like this. Just make sure you look up some parts prices so you don't get a shock. If you can locate parts used somewhere, or find them cheap on eBay and have a little downtime where you can take things apart and try to fix it, you have a great car. If you're gonna take it to a shop and say "just fix it" this will be the most expensive car you've ever owned
Mine has been making the noise for 50,000mi and it isn't getting any worse. When it needs an oil change, it's louder. When it is cold, it's louder. At times the noise is almost undetectable and other times you can clearly hear it.
An option is to just drive the thing until something goes wrong with the engine. My guess is you could put 60k mi on it and the noise will get a little louder but nothing will go wrong.
If it were me, I'd take a quick look to see if I could identify the noise and hopefully it's $500 in parts and a day of your time to fix. If worse, I'd save up for a used S55 engine (easily available and not too outrageously priced) and keep driving the car.
A quick look at eBay shows:
Used (worn out?) engine with 150k - approx $700
Better used engine with 120k - $800-1500
Tested, reputable seller, excellent condition engine with 70k - $2700
I would drive the car and buy the $800-1500 engine and drop it in if yours goes bang.
You could end up with an S55 AMG with a used engine in it for $9000 all in. That's quite a good deal in my opinion.