IMHO, Orthodoxy is out of question. Byzzies are simply too far to have a sucessful mission in Turkestan, too long from their bases.
Armenians were not quite proselyt.
BUT, Nestorian Turks would be the winning idea. After all, many turks were nestorian before the arrival of these peoples in Midde-East. Have more sucessful nestorian priest convincing enough leader, and you'll maybe have a good POD.
In the other hand, Islam is a powerful vector of integration in the Middle-East. You have to find a way that turks would be anyway out of controlling the Abassid throne, to be beaten by the persians, and you will have christian nestorian turks as a far less powerful nation in Middle-East, maybe in OTL Kurdistan, or between this region and Caucasus.