As for the wiring I did start out with some problems with the factory toyota wiring so I eliminated every circuit that wasn't needed from that and left the lighting and dash circuits.  I put in all new wiring for the alternator and used an aftermarket relay to cut the coil power when the engine shuts off.  The glow plugs harness and relay was pulled from a mercedes and integrated into the toyota ignition switch.  The toyota starter solenoid wire was just hooked up to the benz starter.  There is a vacuum valve and a solenoid valve that both need to be hooked up to the ignition circuit.  The vacuum valve mounts to the top of the intake and the solenoid valve is on the back side of the injection pump.  I recommend using a relay to energize these two coils so they don't tax the ignition circuit too much.  The toyota voltage regulator that mounts on the fender was removed because the benz alternator has an integrated regulator inside the alternator.  And that's pretty much it.  I didn't use any computer in mine but your transmission may need it if it is an automatic.  I believe the 90 is a mechanical injection system so I don't know why the engine would need a computer unless it has to sync the shift points to the rpm which I think happens in the tachometer circuit.  There is a magnetic pickup next to the flywheel that senses the engine rpm.  Hope this helps and good luck with your swap!