long distances

Are you any relation to Stu Hayim?

120 feet is a lot of boat for you and one deckhand.

Why only one tank of fuel? How fast do you want to get there? Run inside or out? That is a big boat for just two to run. Have you ever made the trip before? Do you own a boat anywhere near that size? This sounds a bit like a come on. If you think you like it, charter one first to be certain.

With a Benetti 120´ you can cross the Atlantic ocean without refueling. But I doubt you can stay at the helm for 300 hours nonstop...

Captain/engineer, mate and two deckhands is what you should look for. And if you are not a chef, get one and a stewardess as well. This is a real yacht.

Driving a boat is easy... it is parking that is hard!

It is possible to run a 100' with two people. Some friends of mine do just that. However, they are quite experienced. Not trying to be critical but judging from the poster's question I don't think his experience level would be up to running a 120'.

Personally, I wouldn't want to be running a 120' without the above recommendation (Captian/engineer, and at least 1 deckhand, perhaps a housekeeping/chef/stewardess) with myself participating as well.

This yacht is more than 18 million USD new and annual running costs is usually 10 percent of this, including crew.

Like AMG said, this is an $18M yacht. Why are you cheaping out on the crew? Or do you not want crew mixing in with family/guests? Can family members handle lines, tie off on a piling, know the pointy end from the flat end? And if you don't want to stop, are you going to run 24 hours/day? You will need several people who can handle the yacht, and rotate regularly. Have you ever navigated at night before? It's one thing going from Port Washington towards New York and the bridges at night, but a different picture running east down Long Island Sound at midnight. It gets pretty dark on the water.
If you want to do it with the least amount of people, then you can probably get it done with three (captain/deck/engineer) but you'll have to stop nightly, cook, rest and do it again the next day.
Sorry, but this is just not adding up.

I actually asked something similar to this a little while ago here:
http://www.yachtforums.com/forums/general-yachting-discussion/7624-yacht-you-can-drive-yourself.html

I too wanted to get a boat in the 100 foot range that I could drive, but after listening to the recommendations and doing more research, I now know it's not only incredibly difficult to do, but it's also rather dangerous. In fairness, I really didn't know anything about boats when I asked that, but I've spent a few days reading these forums and researching online and the very thought of running a 100' alone now seems absurd. I'd go with what Norseman said in my thread: anything above 50 feet requires a crew.

On another note, would a $20 million boat really cost $2 million a year to run? Is that just fuel + labor?