Great, thanks.
I see you are importing some 2e mechanics, such as "half damage". IMHO this isn't going to be easy to balance.
Better to stick with 3.5e stuff like "Energy Resistance 10 (fire, cold, acid and sonic)". That way it's not limited to Evocations, and they're still vulnerable to electricity.
For the electricity side effect, why not create some custom spells for the setting? If the Mechanus dudes were the world's BBEG for a long time, they may have been brought down by spells specifically targeted at them.
For example:
Shocking Strike
Evocation [Electricity]
Level: Druid 1, Sor/Wiz 1
Components: V, S
Casting Time: 1 standard action
Range: Touch
Target: Creature or object touched
Duration: Instantaneous
Saving Throw: None or Fortitude partial; see below
Spell Resistance: Yes
Your successful melee touch attack deals 1d8 points of electricity damage, +1d8 per 3 caster levels (maximum 3d8). When delivering the jolt, you gain a +3 bonus on attack rolls if the opponent is a Construct.
In addition to the Electricity damage, a Construct who takes damage must make a Fortitude save or be Stunned for 1 round. This effect violates a Construct's usual immunity to being Stunned.
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If you wished, you could replace all the regular electricity Druid and Sor/Wiz spells with anti-Construct versions. IMHO that would change the setting significantly, but in a very cool way.
Then you could just use something closer to the relatively well-tested Warforged (with the extra energy resistances, plus feats to boost their energy resistances at higher levels) and your setting would still reflect the race's history well.
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Another alternate mechanism would be to give them Energy Resistance XX (fire, cold, acid, sonic) and also give them Electrical Vulnerability (+50% damage from Electricity effects, whether they save or not). There are plenty of critters with Fire or Cold vulnerability; it's not much of a stretch.
Cheers, -- N