Sounding good so far.
For ability scores, taking a Large viper...
Str 10, Dex 17, Con 11, Int 1, Wis 12, Cha 2
...and applying the zombie template (Abilities: A zombie’s Strength increases by +2, its Dexterity decreases by 2, it has no Constitution or Intelligence score, its Wisdom changes to 10, and its Charisma changes to 1.)...
Str 12, Dex 15, Con -, Int -, Wis 10, Cha 1
Since save DCs will probably be Cha-based, bump that to 11 or higher?
Seem reasonable?