Digital transmission of composite video
There are commercial chips that will digitize an NTSC (or PAL) signal for you a 8 to 10 bits of precision, but they typically do it at the standard rate of 27 MHz, giving a raw bit rate of 216 - 270 Mbps.

But after you have this digital signal, it is relatively straightforward to throw away the parts you don't care about (the chroma channels, the audio, etc.) and decimate the sample rate. You can even compress the data using an H.26x encoder to get the data rate down to 1 Mbps or less.

There are SoC chips with hardware video support that can do this sort of thing, or you could look into using an FPGA.