Glad to hear your enjoying it, terraform is fun but can get expensive in metal real quick for big projects. It's good for protecting explosive structures. You can ctrl+shift+click one of your buildings (like geothermals) to terraform a wall around it.
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Based on your descriptions your internet sounds to be about the same speed as mine. Yes, you will have to be patient with downloads as maps take up a lot of room, the new campaign has 29 missions and each uses a unique map.
Anyways point being my slow internet isn't much of a bottleneck when actually playing, ZK is pretty bandwidth efficient and doesn't require fast ping. Assuming you have a nice GPU (amd or nvidia) and you can wait for all the content you should be able to play fine.
The greatest RTS of its kind, ZK takes all the best parts of Total Annihilation/Supreme Commander style games, and throws out the need for extensive eco basebuilding or teching up. It's possible to make maps with an extremely fine level of detail, with lots of room for modern lighting, normals, and other shaders. Extensive community contribution framework. A joy to both play and contribute to.