Andy I disagree with you rationale for not combining defensive and offensive weapons. You can still make defence cheaper than offence in a combined weapon situation by just applying a blanket percentage advantage to defenders in any battle. So for example, when a unit is being attacked, it could have say a 20% advantage compared to the attacking unit. That advantage would make defence cheaper by 20%. There is no need to have separate units. It just doubles the logistics for the military.
Similarly, there is no need to have 4 different land units (tank, arty, jeep, armoured vehicle). They are all pretty much the same. Make them one unit and just call it Combined Arms Ground Force or something along those lines.