I really don't like syncretists. (fixed)
One of Christianity's big points is that salvation can only come through God, specifically the form of Jesus of Nazareth. To try to save yourself without God is essentially denying God. Buddhism explicitly makes the statement that humans can discover the truth and thereby save themselves, without teachers or gods - and goes so far as to say that gods can't really help with enlightenment. Syncretists would have you believe that both religions are simultaneously correct, describing different perspectives of the same truth as the various blind men are describing the same elephant. Similarly, Mohammed was the last prophet... there just happen to be many other prophets (J. Smith, Baha'u'llah) of God who came later. It's okay in that it doesn't say that any religion is wrong, it just recognizes that the people who say that that religion is wrong are also correct.
It's doublethink and compartmentalization of beliefs at a massive level under the banner of tolerance.
Buddhism is essentially an atheistic religion; at least it's one where believing in gods is not a part of the mainstream belief.
You mention combining Buddhism and Christianity, in spite of their contradictions. I think of modern Christianity itself as syncretistic. In the Christian New Testament, it's presented as the fulfillment of Judaism (i.e., the prophets were all right about the Messiah, even if no one understood what they meant, but now we can explain it all to you--they all point to Jesus). Every miracle attributed to Jesus of Nazareth was done in some form by a Jewish/Old Testament prophet (except, perhaps, for raising himself from the dead).
But look at the story of a man who was the son of god and a human virgin who died and raised from the dead. That’s both Jesus and Dionysus/Bacchus (who came before Jesus).
The modern day Christian version of Hell is largely from Greek mythology and Dante's Inferno, not from the Bible. The wise men seeing the star and reading it as a sign in the heavens, and the book of Genesis specifically saying that stars are for signs, likely come from the religions of Persia and Babylon, respectively (the Jews generally weren't into astrology/astronomy).
Then the church in the European Middle Ages ironically added the teachings of a pantheistic Aristotle, and fought science because it disagreed with Aristotle (although some of the great works of science were, ironically, done by Christian monks).
But if you really want to find a syncretistic belief system, look at Discordianism. It borrows from all over the place. The difference is, Discordianism not only admits the borrowing, but accepts its own internal contradictions, something most religions don't do.
Personally, I think that Discordianism makes the most sense of any religion, because it admits its own nonsense. At least in some sense.