Running the 460V 60Hz motor on 400V 50Hz is questionable. It will run at 5/6 the speed, i.e. about 1450 rpm which could be corrected by the belt drive, but it is not going to develop 15hp on the lower voltage.
Running a VFD into a panel is a No, No; VFDs have to be wired directly to the load and any switching has to be on the input side. Switching the output side of a VFD is probably going to destroy it.
The natural gas powered genset would work, I use a diesel 15kVA unit to run two CNC machines and it works just fine.
But why not consider getting a 15kVA generator then when you are in Poland get a 20hp 400V motor to drive the generator. The reason I suggest overpowering the motor is to take into account starting surges from your machines.