Riding from Ger to JPN is no problem, I did it 2015.
Although it wasn´t easy, but doable. Riding in Japan ist gorgeous.
If you want to socialize, contact the Gajinriders.
Buying in Japan is easy, Red Baron ist the biggest dealer, they can provide any bike, but registering is not possible, unless you are a resident (which is also almost impossible). Entering South Korea on an alien bike is almost impossible unless you are US citizen and entering with a US registered bike.
Leaving the bike will cost you the customs fees and the taxes, in most countries, so not advicable.
I shipped my bike from Japan to Bremerhaven for 1290€, and it was easy, the only thing which was easy in Japan, but the company was german and had a branch in Tokyo/Ginza.
No more harbor, shipping storage or other fees, all incuded. Thats very rare in the shipping business.
My suggestion would be: ship the bike from Ulaan Bator via Oasis, and fly to Japan and rent/buy a bike there. You have to have your international drivers licence translated by the JAF (jap. ADAC). DBS Ferries has an awesome service. I had a luxury car with driver from them for 70€ to get my drivers license translated which was another 30€.
And you`re free to ride up to 6 months in Japan.
Riding is awesome there. I visited Kyushu, Honshu of course and Hokkaido. But I wouldnt go there on my own bike again. The Jibaiseki (?), the insurance for injuries of third party is mandatory, but its hard to get it, as it is not totally legal for germans to get one. Contact me if you need one, I know a place where its easy to buy one, if you ride without one and get stopped by the police, youre screwed, as it is the first thing the cops ask about.
And don´t carry any knives or other weapons, they freak out about that.
Keep it with your camping gear.f you want to do it anyway, contact Yuri in Vladivostok, its about 1000$ for you and the bike to go to Japan, and another 100 or so for the JAF and landing.Yuri is professional and does a good Job, although some destinations can get very expensive. My friend shipped to Vancouver, and it was crazy expensive as the shipping company charged more...step by step, as reported here in many cases. But this wasn´t Yuris fault. So my advice would be: not on my own bike, and not with a rented car.
Max speed is 60, often 40 km/h, and the highway is really expensive. The most expensive was 1,50€ for a stretch of 180m.
On a bike the roads are big enough and you can ride vey safe, with a car it is a drag.