The challenge is to make Buddhism the predominant religion of an ATL European state. It must survive to the present day. Bonus if it becomes a regional/great power.
Well, Christianity was radically different as well, but it managed to break through into the Greco-Roman world.IOTL, Buddhism was more assimilated to an obscure mysteria or a philosophy than a religion by Greco-Romans. Which was the best you could have, granted they were really cautious about foreign cults they couldn't assimilate or identify to their own (and Buddhism would be radically different from these).
Well, Christianity was radically different as well, but it managed to break through into the Greco-Roman world.