I see so you want to have blank chassies and then Add what weapon you want to them later.  That seems like a extra un needed step to me but it would also add a Weapons plant that would build the turrets.  I think that would work tho. 
 That is how op2 does it. Each chassies has a cost and so does each weapon.
And i didnt say  the RPG would have a attack of 2 and a def of 3. I was saying the RPG is not 3 times as powerfull as the microwave. 
Say a weapon has a value of 10 then a weapon three times stronger has a value of 30. 
What i was saying was you cant use value 1 for the weakest value because the weapon above it is not two times as strong.
I didnt suggest HP i just suggested two values for HP, full HP and half HP (damaged).
I wanted to have garage's and repairs for vecs because thats like op2.  And yes they would hold more and 6 units, remember im not using op2 buildings, im using buildings which are much larger. 4x4 or 5x5 squares in op2 terms big is each building.
I dont plan on including spiders/scorps and there is no field repairs. Well maybe i could add it later. Each spider you have increaces your attack/defence strength beause units could last longer if their geting repaired.