What capacitor to get for relay coil?
Paralleling the coil with a capacitor can work the driver quite a bit harder (possibly causing it to fail) and may cause a brief dip in the 24V supply- which could cause glitches. If the driver is relatively slow or the current is limited it may be useful to reduce EMI from the coil, however the contacts usually dominate the noise and in any case the driver circuit will likely determine how big you can safely make the capacitor. 

Chances are a reasonable value would be some nF and best served by a ceramic capacitor of adequate voltage rating.