Hydrogen, the backplate and wing system has three parts: The plate, the wing, and the harness. You can, within very broad limits, enjoy a variety of choices for each part. Plates come in various materials and sizes. Wings vary as to whether they are made for single tanks or double tanks (despite what some will say, you really can't buy one wing that will do both adequately) and within each group, how much lift they offer. Harnesses can be simple webbing or have padding or clips.
What you have to decide first is the environment in which you will be using the gear. If you are diving in Canada, you will most likely want a steel plate, and you may actually want something like the DSS steel plate, to which you can attach lead weight plates. My husband uses them and likes them a lot. On the other hand, if you are planning on warm water diving, you may not want steel, because you don't require the ballast.
You will want to buy a wing for single tanks to begin with. Such wings come in sizes from 17 lbs of lift (warm water aluminum tank diving) to 35 pounds or so (large, steel tanks in cold water). They vary in how they attach to the plate, and whether you require an additional piece of equipment to stabilize the tank (single tank adapter). They also vary in materials and in access -- with some wings, you can easily replace the bladder if you have to; with others, you have to send the rig back to the manufacturer for repair.
I think most of us who are using backplate setups would recommend starting with a simple webbing harness. If you find it unmanageable, you can always spend the money for a "comfort" type harness later. But it's been my experience that most people who spend the money for the complex harnesses, eventually go to simple webbing, anyway, so the extra money was wasted.
Within your budget constraints, I would recommend a DSS setup, especially if you are diving cold water. This is for three reasons: One, Tobin George (owner of DSS) will walk you through the process of selecting equipment that will match your needs. Two, you have the option of the weight plates, which are an elegant solution to ballast requirements in cold water. And three, should you ever have issues with the DSS gear, their customer service is incredible.