DO NOT take power from the starter. Take it from the battery.
If the battery is relocated to the trunk (boot) as your diagram indicates, that makes your wiring a lot simpler.
You just need a line that senses ignition state to trigger amps and/or PC power supply. You can probably get this from the stock radio harness or a cigarette lighter (if your car has one).
If there are no other options, you can get your ignition line from the ignition itself.
This simplified (and shorter) power run also makes runs for audio lines easier, since you don't have to worry about power lines and audio lines running next to one another, potentially introducing noise into the system.