The tax information I can answer as I lived and flew in Japan for 14 years. You're tax free for the first 5 years and are requried to pay taxes after that but the tax authroities really never check. I did not pay any taxes the entire time I lived in Japan.
There are some questions on training and that is another story. It is hard and the physical is "astronaut physical tough". But after the first run with the doctor there are several doctors, one at the airport that can give a quicky physical. After you're through the training purgatory you will find the line work is just like any other.
I enjoyed my time in Japan and only my age kept me from staying and believe me my wife and I did want to stay.
If you managed to get hired, live with the Japanese instead of in and around the gajins (foreigners); we made many Japanese friends some of which still visit us here in the states. A bonus to that is we learned Japanese and even could read two of the three alphabets. All in all it was a great time in our lives and if picked you will be a very lucky man. If could go back I would in a heartbeat, alas my age kills it (ahem)