800khz digital signal amp, transistors and biasing, opto-isolator
MOSFETs might switch faster than BJTs.
But to determine how far away you are from the edge, you have to find out where the edge is, i.e., increase the frequency or introduce more noise.

Anyway, that circuit has no isolation, and you are correct that you'd need a high-speed optocoupler for this.
The H11L1 would be rated for 1 Mbit/s, but only for square signals, so it's probably beyond the edge. The 6N137 with 10 Mbit/s capability would work just fine.

The following circuit is stolen from the MIDI specification, and implements a 5 mA, active-low current loop:



(The resistors R1..R3 are distributed this way to protect against short circuits at the cable's plugs; if you have a fixed connection, you might be able to combine them into a single resistor. The diode protects the 6N137's LED against reverse voltage; in a noisy environment, you probably still need it.)