You need an ECU chipped to run Hondata. That way you can plug in the s200 box to the ECU. As for the obd harness, it depends on what ECU you use. If you use an obd1 ECU then yes, you will need the harness. If you use an obd2 then you are good
You cant use a stock ECU as you wouldnt be able to plug in the Hondata