Look at the factory wiring diagram for whatever throttle body you're planning to use, and check out the fuse size dedicated to powering the injector assembly. That's by far the highest current draw you'll have. The keep-alive circuit on the computer will be fused at a much lower capacity than the injector quad drivers. Virtually all the sensor circuits will be measured in milliamps, so the power supply and the injector coils will use the majority of the current.
Since original-equipment systems get by without a fancy isolated-power setup, I can't imagine why one would be necessary on a retro-fit system. The reference voltage to most of the sensors is set at 5 VDC and supplied by a filtered voltage-dropping circuit, and the injectors are designed to run on straight battery voltage with the quad drivers providing the ground. The magnetic coils in the injectors dampen out most voltage spikes by inductive reactance. If you wanted to smooth out any spikes with a big electrolytic capacitor in parallel with an inductor wired into the input circuit, that might be OK, but I believe even that would probably be overkill.
Jerry
