To get any kind of meaningful advice, we need to know the conditions under which you dive. Water temperature, wetsuit thickness, weight carried, your present height/weight and things like that.
Personally, I think you would do well to start with buying a good regulator but that's just me. First I breathe, then I float. That kind of thinking...
Do some research on BP/Ws (backplates and wings). You will find an aluminum backplate for $50 from LeisurePro. A stainless steel plate for $97 if you do cold water diving. You can build up the harness for less than $50 and a HOG 32# wing is around $200. All brand new. That's at least $100 less than the Knighthawk at LeisurePro and probably a couple of hundred cheaper than at your shop.
I'm not a fan of buying used from eBay but there is a nice BP/W there today for something around $200. The "But It Now" price seems too high.
Talk to people around your dive shop. I picked up a nice wing for my wife for $100 because the diver had changed to a different model. No reason not to buy it so I added the backplate and harness and she it good to go. You may find other divers have a lot of left over gear that could be purchased cheap. It won't be the latest and greatest stuff but in many cases it will be quite good. Even shops sell their gear from time to time. Sure, it will be old and well worn but it's a start.
Check the used equipment for sale forum here on SB.
Richard