Give this:
West Marine: West Advisor
a bit of a read... it lays out the various ways of plumbing up this equipment. It's pretty nice to have the option of overboard discharge or
holding tank pumpout in the appropriate open waters, and some of these layouts allow both.
If the laws are more restrictive in your area perhaps there's no point in even building an overboard discharge, but even then as an emergency measure it may be worthwhile.
Good luck, at least you're starting from scratch, not fixing someone elses botched job.