Hey Uwe, I'm Heinrich a German fellow.
Last year I traveled the US for six months. I started in San Francisco, went to the east coast and then back to SF. Wheatwhacker from this forum helped me buying a bike and it was overall a wonderful experience. I'm going to visit him beginning of May in Ireland, as he returned to his motherland during my trip across the US last year.
At least in California it's not a problem to buy and register a bike as a foreigner. You pay the registrations fees (~10% of what you bought the bike for) cash at the DMV. Progressive is your best bet for getting insurance without an American driver's license. Simply paid by credit card. For my V-Strom 650 it was about $700 for a year of coverage. Including collision coverage, which would be comparable to the German Vollkasko. This paid of in my case
. The only thing you need is an address in California that you can use to register the bike. Nobody cares if you really live there.
I can give you contact information of people over there that might be willing to help you. Or better, directly ask wheatwhacker as he knows a lot of guys in California. Finding someone whose address you can use to register the bike will be no problem at all.
And if you're lucky, one of those might do the same thing, wheatwhacker did for me about a year ago. I watched craigslist for a bike that seemed reasonable for my trip. When it popped up, I told him to go for it, he went there, took a look and picked it up for me. When I arrived in San Francisco, it was already waiting for me in his garage. When you're done with your trip, you can just sell the bike again.
Just remember, if you want to stay in the US for more than 90 days, you have to get a visa. Not a big deal, you just have to visit an American consulate und pay about 120€.
Ther than that, plan ahead in terms of when you want to visit. The western US has a lot to offer. From high alpine regions, especially in Colorado, which I highly recommend visiting to vast deserts. The alpine regions are best visited between July and September. Many passes are open only by July because of all the snow there. I came through Colorado last year in October and I hit the first snow. Your at almost 4000 m altitude, so riding season is limited up there. But it's awesome! On the other hand, avoid deserts during summer. Riding is not much fun and it's really dangerous. Expect 40°C+ and no shades for many miles.
I hope this was helpful. Enjoy the trip, there's a lot to see over there! If you have any questions, don't hestitate to contact me.