Well, you mentioned you want to use engine’s lighting effects, so my first thought is: avoid any visible light sources on the sprite itself.
I don’t know if this is possible in godot, but 2d normal mapped sprites can create some cool effects (quick google example: http://www.alkemi-games.com/a-game-of-tricks/ ).
It really depends what you need, maybe you want one key light like in the linked example above (on the character) and just add lighting from normal map as the “rim light”. Or just use engine’s lighting as the main source of light with combination of the normal map (normal maps would have to have more details than the “rim” then), like in this example: http://4.bp.blogspot.com/-jc_sSbCebV8/VDf45Z1qARI/AAAAAAAABaE/FPiTQnZKqvw/s1600/Sprite_DLight_Maps_Preview_Doc_Pilgrim.png
The ambient occlusion in the second case is important, because with uniform lighting (like simple one color environment neutral light) you almost have no details - AO would bring back some of those details back, while still keeping the model lit, without any main source of directional light.
So the it really depends on what you are going to use and how. Maybe directional light is ok, maybe you’d want to avoid it and use flat lighting.