“Y not” said: “So, as I see, Buddhadhamma fits perfectly with the tenets of the Upanishads.”
It is actually the other way around. All these concepts to Upanishads or Hindu Vedas ORIGINATED in the previous Buddha Sasana of the Kassapa Buddha, who was born before the Gotama Buddha.
There have been four Buddhas in this Maha Kappa, or within this life cycle of this Earth (the Solar System), including the Gotama Buddha. These Buddha Sasana (or the Ministries) last only a certain short time compared to the age of the Maha Kappa (which runs to billions of years).
Gotama Buddha Sasana will last only 5000 years (2500 years more). Then it will fade out, just like the Buddha Sasana of the Kassapa Buddha before that (which is said to have lasted 80,000 years).
What happens is that at the end of a Buddha Sasana, the true meanings of key words are lost. But the Vedas had kept the mundane meanings of the concepts revealed by the Kassapa Buddha. That is why most of these terminologies were already there when the Gotama Buddha (Prince Siddhartha) was born. But they had lost the true meanings.
For example, Anapana bhavana was there as breath meditation. Saptha Bojjanga, five or eight precepts, metta bhavana, etc etc were all there just with mundane interpretations when the Prince Siddhartha was born. Of course the concept of kamma/kamma vipaka were there too in the Vedas. The Buddha has mentioned this in several suttas. At some point, when I have time, I will write more. But it would be nice if someone can post such suttas here for others to see. I simply do not have time to look for them at this time.