Hello guys. I would like to recomend to focus on Automotive ethernet, standard 100baseT1 (BroadR-Reach). Example of PHY nxp TJA1100,
It can be used in place original PHY of regular 100baseT Ethernet PHY. It is much more robust against EMI than PC version. It is already used in BMW. I can imagine nice, small kit that has on one side MII to interface MCU and on another IEEE802.3bw (3rd name for this ETH standard)