There isn't very much difference is plates outside of material, it largely comes down to how much weight you want on your back. My FredT SS plate is about 6lbs, my Halcyon AL one is closer to 2lbs.
Wings vary, I'd personally go with a circular wing, they are easier to dump air from, again people can debate how much lift you need, but it's probably less than 30lbs for almost any singles rig.
You'll have to choose whether you want a Single Tank Adapter, I use one, but it's mostly for the added weight. If you choose to go without one, you'll have to get a wing with some sort of stabalizer built in.
I use a simple Webbing harness, I have the Halcyon "kit" on my Al plate and one I put together from parts on my FredT plate. The only thing I'll note here is that the webbing on the Halcyon "kit" isn't as heavy as the generic rolls of webbing I have, and as a result I actually prefer it. Honestly though there is sod all difference once the webbing is wet so I wouldn't conside the Halcyon worth the additional cost.
My first BP&W was entirely Halyon, but they are expensive and there are comparable products from other manufacturers, I'm sure others can chime in.