Capacitor across motor vs capacitor across motor and transistor
The MOSFET you chose will probably fail. Using a 30V device on a 24V system is asking for problems. Go to a minimum of a 60V MOSFET and use a UIS, avalanche rated one.
R4 is very weak for what you need to do. I would suggest something in the 10K range to swamp out any leakage in the Arduino.
If the MOSFET is properly rated, the flyback diode is not needed but make a space for it incase you have to sub a MOSFET.
Putting C2 in parallel with C1 will help. I do this a lot with electrolytics. I double up - each one is the needed size so when one fails you don't notice it.
I would suggest a couple of 220nF capacitors across the motor but that is application dependent.