I bought the Dive Rite SS plate with the TransPlate harness and the STA (only so I don't have to change a thing when I eventually go doubles), and have been very happy. My LDS put it all together for me, however, so I guess I'm somehwat useless on the setup questions.
They didn't charge me anything extra for keepers and D rings, however, so unless they were beintg extra-super nice (which is possible) I suspect the rig comes with everything you'll need other than a few simple hand tools. (wrench, pliers, knife, lighter) Give yourself plenty of time to set it all up, because it took them quite a while and they've done it a few times!
But yeah, the main components are plate, wing, harness, STA (optional) and cam straps. I also like the Dive Rite cam straps better than anything but ScubaPro's. After that, you're looking at deciding on weight belt vs. weight pockets, and any other accessory pockets you might want. (I use a DR mask pocket as my main pocket.) Pockets are the one thing I miss from back when I wore jacket BCs...

But yeah, the main components are plate, wing, harness, STA (optional) and cam straps. I also like the Dive Rite cam straps better than anything but ScubaPro's. After that, you're looking at deciding on weight belt vs. weight pockets, and any other accessory pockets you might want. (I use a DR mask pocket as my main pocket.) Pockets are the one thing I miss from back when I wore jacket BCs...