Not a bad idea either. There's just one thing I don't like about the background, and it's them deciding that they don't want to fight another man's war. No, they believe the humans to be weak. They believe that they are superior, and don't need to take orders from humans. That would make more sense in terms of that.
Story has never been my strong point. =P
As for buildings, I actually thought about doing it like Power Towers TD just thought it might take up too much base space. Though it would look much cooler with everything connected and I guess you can scale the buildings down.
For the units, it would be nice having just a few empty units that can be fitted with addons that make them into certain class types, like heavy weapons, or repair module or scout radar. And as the machines advance more, like mutants, you could upgrade they're barracks to produce the more advanced tiers of machines so they don't have to upgrade one unit 4 times to get it to be more inline with the rest of their troops.
But yes, being able to sort of customize your troops with addons would make them feel different then the rest of the races and be pretty fun. Probably too much work but being able to say...
Empty Chassis - Addon (Light Weapons, Heavy Weapons, Repair Drones, or Scout Radar)
Choose Addon (Light Weapons) - Now you have a few new options (Scout Radar, Repair Drones, or more Light Weapons)
Choose Addon (Scout Radar) - Now more different options (Faster Leg Movement, Improved Scout Radar Range, Stealth Generator)
End up with a Scout Vehicle with light weapons and scout radar and say you chose stealth generator.
Hmm, as awesome as that would be that's a ton of different unit variations lol. Cause you could also have a scout vehicle with light weapons, scout radar and faster leg movement. I guess you could make them abilities too, to cut down on the amount of units needed.
EDIT: Added in unit variations to give you a better idea of what I was trying to describe.
Units
-Machine units should be strong and few. Expensive. Since, they can't be affected by mutant disease and the human projectile weapons won't kill them nearly as fast. (Let's just go with because they have no organic tissue, the bullets would need to do considerable damage to basically shut down a machine unit)
So, you can either focus on hybrid units or make more sturdy unit roles. Basically if you want a good scout unit, you would want to start with scout radar. As the tier 4 Upgrade would be nice for scouts if you start with scout radar. If you don't start with scouts then you don't have access to scout tier 4 upgrades. Actually maybe no access to tier 4 upgrades unless you stick with whatever you chose as tier 1 throughout upgrading. Like...
Tier 1 Light weapons, tier 2 light weapons upgrade, tier 3 repair drone upgrade, no tier 4
Tier 1 Light weapons, tier 2 light weapons upgrade, tier 3 light weapons upgrade, tier 4 light weapons
Alright, on paper this is pretty hard to follow. Though in game it would be simple since you'd basically see all upgrades on your unit when you want to advance to the next tier. It's set up so you could, if you wanted to, have a unit have light weapons, a simple scout radar, and some repair ability.
Light Chassis
(No armor, no weapon)
-Upgrade the defenseless chassis to a tier 1 unit.
Tier 1 - Light Weapons (Adds weapons, small hp boost)
-Tier 2 Branches into - Scout Radar, Repair Drones and Improved Ammo
--Tier 3 Scout Radar Branches - Improved Speed, Repair Drones (No tier 4)
--Tier 3 Repair Drone Branches - Improved Armor, Scout Radar (No tier 4)
--Tier 3 Improved Ammo Branches - Add Armor, Scout Radar (No tier 4), Repair Drones (No tier 4)
---Tier 4 Add Armor Branches - Combat Drone (Improves in all light weapon areas, atk, atk spd, etc.)
Tier 1 - Repair Drones (Adds Repair skill, and some armor, medium hp boost)
-Tier 2 Branches into - Scout Radar, Light Weapons and Improved Armor
--Tier 3 Scout Radar Branches - Improved Speed, Light Weapons (No tier4)
--Tier 3 Light Weapons Branches - Improved Ammo, Scout Drones (No tier 4)
--Tier 3 Imporved Armor Branches - Improved Repair Rate, Scout Radar (No tier 4), Light Weapons (No tier 4)
---Tier 4 Improved Repair Rate Branches - Repair Bot (Improves in all repair areas, armor, repair rate, etc.)
Tier 1 - Scout Radar (Adds scout radar, and slight speed increase, no hp boost)
-Tier 2 Branches into - Repair Drones, Light Weapons and Improved Speed
--Tier 3 Repair Drones Branches - Improved Armor, Light Weapons (No tier 4)
--Tier 3 Light Weapons Branches - Improved Ammo, Repair Drones (No tier 4)
--Tier 3 Imporved Speed Branches - Improved Radar Range, Repair Drones (No tier 4), Light Weapons (No tier 4)
---Tier 4 Improved Radar Range Branches - Scout Drone (Improves in all scout areas, hp, radar range, etc.)
Now with tier 4 upgrades you run into the issue of, is there even a reason for making hybrid units? So, can either do away with tier 4 upgrades or figure out some way to make hybrids more attractive.
Heavy Chassis (Researched later, for tank type units)
(Heavy armor, no weapons)
-Upgrades
Hover Chassis (Researched later, for flight units)
(Light armor, no weapons)
-Upgrades