firstly -
THEY are morons who are claiming it as AMG, its like saying Toyota Indus made a TRD spec XLi corolla.
now for the gauge, get the car to a proper mercedes shop, the gauge cluster is not a simple unit like on a mehran, all the info on it is sent via can bus to the cluster, so if you are missing an input you will literally see an error when its put up on a proper scan tool.
The sender usually doesnt go bad - sounds like someone was fiddling with the fuel pump and somehow damaged the wiring to the module.