The beauty of 24 is for given ammount of work, the cables can be significantly smaller than those needed for that much work at 12V.
Yea 24 will be a little harder on the motor windings and brushes, and if you really load it up, well you have more potential than those components can handle… But in this application, being fed from a slave plug, will have some line losses which will get the 24v closer to the 12V motors ideal, so probably not a bad match up for this application.
The contactors/relays are typically controlled by 12V so swapping them for 24v coils is not a bad idea As they will run hot, but this is probably not going to see a terrifically high duty cycle either…