The emgine wiring loom takes it's power from the connectors on the engine.usually a multiplug/socket.
The engine is connected to the battery via a fuse and switch, if you have the money make these cables tinned copper wire.
Any other bits of kits you want should be conected via some form of fuse and bus bar(Distribution Rail) easiest way is to have a negative terminal block and a multi switch with each switch having it's own fuses.
Personally I'd use Blue Sea Systems switching stuff but if you wqant to use other stuff these links might be useful
Here it comes again, I don't stand a chance
Soul possession, Got me in a trance
Pullin' me back to you - Deja Voodoo