The difficulty with BP&W is getting going. It's easier to buy one all made up. I have the same BP&W as when I dove doubles. Back then I bought the original Halcyon BP (before the blue "H" was on the webbing) and added a 55lb Halcyon wing. It turned out to be way too big, so sold it and got an Agir 38lb wing. I'm still using the same BP&W today.
Mind you, I sold the doubles and switched to a rebreather. The RB is bolted to the same BP, but the d-rings on the shoulder straps were replaced with side-rings because of the front counterlungs. I also added a butt plate and bungees to sidemount my bailout and some bungees to the crotch strap to hold the counterlungs down. Otherwise it's the same BP&W as ever.
For OC diving I wanted the same thing, but separate so I didn't have to convert. I just went out and bought a SS back plate, then 24ft of 2in webbing, some d-rings and a buckle and built my own using the original as a pattern. I also added a butt plate and bungees to allow me to stage my sidemount rigged tanks if I want them. Over the years I had bought another Agir 38lb wing, so had one for this rig as well. The backplate has slots so I just bought two cam-bands to hold the tank.
It's nice diving the same harness, backplate and wing whether diving doubles, single tank or rebreather.
Back to the OP - for your first BP&W it's easiest to buy it complete. You will get lots of advice whether to get something fancy, or padded straps, or quick-release shoulder straps, etc. etc. etc. What you buy will depend on whose advice you take and how you like the ones you try on. I personally prefer simple web harness because it's easy and cheap to build your own once you've seen one, but you make the decision here.
Final bit of advice - find a dive shop that rents the one they sell. Then dive it and see if you like it. If possible, do this with as many types as you can find to rent, because the worst expense for gear is the one you have to do over.