First check if the motherboard has a serial port header somewhere. On my PO, there is a serial port but the connector is not on the backpanel, instead they put it on a connector in the middle of the board
I'm not sure a pci express serial card can work for getting bootloader logs, because it will not be exposed at the default com port address. While we can change where the kernel looks for a serial port, it is not that easy to do forthe bootloader.
Again I first recommend trying the efi boot image, it's more likely to work on this machine.