If you are using a Eachine Racer 250 there is a 5v regulator in PCB and all the wiring stuff is already done.
The motor port already provides the +5v power to the CC3D 'clone' board and in fact the same +5v to the FlexiPort for receiver.
Just plug the iBus receiver to the Flexiport according to the wiring and that's all.
The only difference is the FlexiPort found close to the receiver port in your ER250 - CC3D clone.
You are talking about a current sensor, please note the CC3D do not support current sensor.
18V mean the sensor can support this voltage for measurement.
ER250 motors / Esc only support 3S / 4S battery
Please be more precise about the issues you read in Wiki.
More information ? ^^