18v is the absolute maximum rating for the heater. This is due to watt density. I have confirmed this with the manufacturer. 24 volts would be quadruple the power, and as a result, quadruple the watt density. This would create localized hotspots in excess of what the insulation inside the heater can handle.
Cannot comment on the Youtube video without a link to see the specifics of what he's doing.
Motor brushes are not supplied as spare parts. Whole gearmotors are, and depending on availability just the motors upon special request.
The fans are 12 volts also, so you'd have to swap them too. There is no reason to run the motor, fans, or stall protection board at higher than 12 volts. If more heater power is needed, get a 15 or 18 volt supply and use it to power the PID controller and heater. This has been done and discussed on the forums:
http://www.soliforum.com/topic/12567/in … mperature/
18 volts is double the heater power that is produced at 12 volts. That is enough to get to ~400C, the thermocouple should be upgraded at anything over 260C as that's when the PTFE insulation starts to break down.