This is my basic Necramech build for [Voidrig]. I didn't show any formas here, because it's a build for not so advanced players. Here is everything that you will need in everyday use:
Survivability - Health, Shield, Armor, and Shield Recharge
Ability use - Energy, Strenght, Efficiency
Mobility - More Thrusters
I didn't use any more mobility mods, because Voidrg is used for staying alive longer and killing everything faster. If you want mobility just go with [Bonewidow]. Of course if you add some formas you will be able to put more mods to make the Necramech better at mobility, crowd control, efficiency, etc.