This is a MUCH more complicated issue than your simple account suggests. First of all, distinguishing between Christians and Jews in the earliest centuries of the church's history is very complicated, and the accounts of the church fathers are tainted by polemic and disagreement. Consequently, making many definitive statements about the Ebionites is difficult - how does one "look into" a sect(s) whose attestations and descriptions vary significantly? Nevertheless, the mere fact that many Ebionites affirmed Jesus' status as Messiah (though denying his divinity) would have placed them outside the bounds of Judaism (thus such could be more appropriately called a Christian sect).
Secondly, Deuteronomy 18 does NOT contain a prophesy that the Jews would fall away and God would send a prophet to draw them back. Rather, the prophesy of Deut. 18 is framed by the prohibition against practicing the divinatory methods of their "pagan" neighbors, as Yahweh would supply a prophet like Moses. More importantly, the belief that Jesus was the prophesied "prophet like Moses" was NOT unique to the Ebionites; it originated with the apostles and was widespread in the earliest church. It's evident, for instance, in the thematic links Matthew draws between Jesus and Moses throughout his gospel (birth narratives, lawgiving, etc.), and even more explicitly in John 1:21.