I’ve added base as a linked object to each piece file, just in case I decide to change first design. So I’ll be able to change only one original base file, and others will change automatically.
When I am sure in my desing, I’ll convert linked file to local object. Then combine it in one scene
For materials I also created one file that will be linked to others and in the end, I hope, when I combine it, all will be work as I want 
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Sounds like you have a plan and all under control.
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