1. I dont think you can power an external monitor via the lcd ribbon because it probably doesnt provide enough juice and the right voltage. plus it would be a PiTA to rig a cable to do that that would remain secure enough. chances are ud screw up the mobo in the process.
2. you may want to consider a power supply such as a carnetix or opus to protect your laptop and especially monitor from power spikes during cranking. otherwise your components will die out faster. if you go that route ull also get the nice power on and off and make sure your components dont drain your car battery if you leave it sitting around for a few days.
IMO, spend a but more but do it right