Ok, going to answer selective questions, however, almost all of this is answered elsewhere.
3) Yes, unless you want to flame your car! How many amps... 150W / 12V = ~13 Amps. Most accessory power suplies use 20 Amp fuses. This would ok.
2) Gauge wire for 20 Amps. Run the black (-) wire to your car chasis. Should be as short as possible.
4) I'm planning on using a feed from the cigar lighter as this is switched. Also, the OPUS will not draw very much over this. To use this line, buy an add-a-circuit which enables you to connect directly into the fuse pannel.
5) Planned on running this off the Cigar lighter power from the same circuit as the OPUS trigger line is connected to. Obviously there will be a fuse in there as well. This for me is ok as it is rated up to 20 Amps and the Lilliput does not draw much. Also, see the Mod for auto power on for the Lilliput.
6) Would avoid using such devices ;-)