Also, use balanced signalling for EMI reduction (downside: needs twice as many components), or route your power+data through coax.
Direct serial would seem unfriendly, given the low cutoff frequency required. You'd probably choose a DC balanced line coding, like Manchester.
Which, in turn, would probably be easiest to generate with an SPI channel at twice the data rate. That is, in software, encode the input data (e.g., 8 bit bytes) into two bytes with Manchester coding.
A modulated (ASK, FSK, BPSK, QAM...) carrier may be better, and certainly easier to filter (both for transmit, receive and EMI purposes). Downside, the circuit gets more complex, and you'd want to look for an integrated solution instead, but I don't know offhand what would cover that...
Tim