The steps are as noted in the thread you linked to, but to answer your questions.
First step: register myself at a city hall where I would have addressee.
What kind of an address will I need?
Could I use address of hotel I'll stay?
I assume an address where you will be staying will be fine as long as you can receive mail there. I wouldn't put a hotel name on the application, though. You have to go to the city/ward hall for that address. Or ask someone in the Japan Community
if you can borrow an address.
Second step: get a Certification of Registered Matters (Toroku Genpyo Kisai Jiko Shomeisho). Document needed to prove your address.
Where can I get this document?
At the the city/ward hall where you register, as it says in my previous post of many years ago.
Third step: buy and register the bike
Fourth step: Get international registration from Land Transport Office.
Fifth step: Get international license plate from JAF.
Actually, I think someone else is issuing them now, but JAF will probably help you with the application.
Sixth step: get somebody to deregister the bike after I leave the country.
Yes. Leave the original Japanese number plate with that person when you leave Japan, and put on the international number. The Japanese number plate must be returned when deregistering.
Final question - what do you think will be the added cost to the purchase
of the bike (insurance, registration etc)
If the inspection has not expired, third-person minimum coverage will come with the bike. If inspection is expired, it will need to be re-inspected at a cost of about 50 or 60 thousand yen.
Note that this is all theoretical from experience with individual processes. I have not heard any first-hand accounts of the process. You may find it does not go as smooth as you would like. Someone in the Japan Community or at the local motorcycle group World Touring Network-Japan may be able to give you a hand.