Your best bet with this is to take a regular Moon Elf and add either the Fiendish (ECL +2) or Half-Fiend (ECL +3) template.
Also, in Monsters of Faerun, they describe a specific type of Tiefling called the Fey'ri. They are descended from elves and demons. However, they are considerably more powerful than normal Tieflings. (WotC hasn't specified an official ECL for them yet, but I'd guess that they are somewhere around ECL +3 or +4). You might take a look that those.
If, for some reason, you were to stick with your original plan, I'd make it ECL +2. Tieflings, by themselves, are already ECL +1. Adding the Elven or Half-Elven traits on top of that would certainly raise the ECL modifier.
As for favored class, it would depend on where you were brought up. If you were raised by your elven parent, your favored class would be Wizard. If, like many Tieflings, you were shunned by your elven parent and forced to scrounge out an existance by yourself, your favored class would probably be Rogue.