A few notes:
A new backplate and harness costs under $150 from my favorite online dive shop:
More Options! & Now Black Hardware! Hogarthian Harness (Dir) for Backplates or
Hog Basic Hogarthian Harness w/ Backplate , Piranha Dive Shop |
Call them and they will be happy to walk you through the options. You'll also need a wing and a pair of cam bands. Get a wing with slots for the cam bands to pass through. 20 to 30# of lift are fine with a 5mm suit.
Get a steel backplate. You will not be overweighted with a steel plate and a 5mm suit, even if you dive with an HP120 (steel) tank. Aluminum plates are meant for those who are desperate to save 4 or 5 pounds when packing their gear for flights. They are superior in the water only for niche uses like steel doubles in warm water.
With a steel backplate, you will not need much additional weight. If you don't want to bother with a separate weight belt, a pair of trim pockets on your harness waist belt works well. Set them back next to the plate and they will be out of the way. I use the following pockets:
XS Scuba Single Weight Pocket . You can get more expensive versions with built in dumps, but I don't have any problem removing the weights from the pouch from the top.
From the tone of your post, I think you'll enjoy the videos from Achim Schloeffel of ISE. I'll link to a couple of relevant ones since you plan on diving a BP/W and longhose reg configuration.
Finally, take a look at the following. Actually maybe start with this one so you can see what trim and buoyancy control really means. These are all relevant for the OW diver except the couple of canister light skills.