Whether or not you need LOC's depends on your amplifier. If you choose one that can handle balanced inputs in the 5V range you will be covered. Other alt is to get LOC's and not worry about it

Yes, it should be as simple as what you indicated - tap the existing headunit outputs, run them to the amp, and then from the amp to the existing factory wiring.

Ground loop isolation and eq'ing is down to personal preference. If you like the sound then you don't need either. If you have a whining then you might need GLI's...

d-