Seeding the tank with live rock means that whatever came in on the live rock can find new and more places to grow, so you get more life growing. More filter feeders filtering, more scavengers scavenging, etc.
What fish are you thinking of so far?
Every sump is different, but they are definitely worth the work. Mine has three sections. The first is where all water going into the sump goes (from the display, skimmer, reactors, and auto top off). It is also where any additives are added. The next section is the refugium with chaeto algae growing in it, some oolite sand as a substrate, some Xenia, a few pieces of rock rubble, and a heater. The last section is for all the pumps (return to display, skimmer, and reactors) and the sensor for the auto top off. Your sump doesn't have to be this complicated, add what you need for your tank, not what you can find or see on other aquarists' sumps and tanks. If I did it again I would use dark glass for the baffles so the light stays in the fuge and doesn't grow algae in the other sections.