First of all, it makes no difference to the NEC if they're Amish or not. :angel: I mean, maybe someone won't enforce the NEC for political reasons, but we are not experts on that here.
If they are buying separate components and materials (panels, charges, wiring) and integrating them themselves, and it is an installation not a temporary setup, then the NEC applies. I'm assuming Michigan has legally adopted some version of the NEC as most states do.
If the units they are buying are listed self contained products that require no field wiring, then the NEC doesn't apply. Or at any rate it doesn't really have anything to say about how they are installed except requiring instructions to be followed. Same if these are temporary setups that they carry with them or move around frequently.