You may get by with your adapter, but it will be close. The efficiency of your buck converter will have a big impact on this. You can use a buck converter to generate the 8.5V for the motor, this will be adjustable giving you "torque" control. Then use a smaller buck converter to again create something in the 8-9 volt range to power the Arduino via Vin. Reason that will give you additional power filtering on the supply. The sensors can be powered via the Arduino +5V pin. You can control the motor (bidirectional I think) with a BTS7960 bridge, not very expensive. If it is only single direction you can use a opto isolated MOSFET switch available for a few dollars from your favorite china supplier. This should get you going let us know how you do.