Any decent auto loan is ~2%. take it, make the absolute min payments. Invest the rest.
This is why I never buy cars outright. "paying off your car" tends to be a financial mistake. Always loan against assets. Never use your own money, etc. etc.
You need the calibration file from the ECU. It's specific to -each- ECU. Includes things like the key programming, VIN, and mileage (among other things). You need the calibration file from the factory ECU.