There are slight differences between some gear. If you want to go GUE there are specifics around which D-rings, where they are placed, etc. So a Halcyon harness is going to have all the right bits. However, even if you buy a harness with not quite the right hardware, you can often change up a few pieces and still be configured correctly.
Stainless steel isn't no-stain steel. So it will rust if you don't take care of it. That said, there are different kinds of stainless steel. If it starts with a 3xx then it is nickel-chromium but 304 will be slightly different from 316 (both can be used for back plates). Again, SLIGHTLY different. The 304 is most common in the USA. The 316 is surgical or marine grade stainless steel. Supposedly better at resisting corrosion than 304. When I was pricing out plates I found the 304 plates were typically less expensive (more commonly available = quantity = reduced cost?). So if you are looking to save a few bucks the 304 steel plates should be a little cheaper.
My last BP/W I bought some webbing and all the hardware then strung the harness myself. Knowing how to do this, selecting a stiffer webbing, etc. will save you a few dollars. Some harness will cost more because you are paying for the branding. Some will have an extra feature, e.g. snitch system allows you to quickly loosen or tighten the harness or deluxe harness have buckles you can open and close the shoulder straps (some feel this is a failure point; others like being able to set the harness up to be tight and take it off my releasing a buckle). These extra features will cost you more.
Different size wings will cost differently. You want to make sure you pick a wing that works for you. I like donut wings because I can roll air bubbles around to shoulder or butt dump valve depending on which orientation I am in. You might want to borrow/rent different style wings to see what you like.
Having a built in tank stabilizer means you won't need a single tank adaptor (STA). This will reduce cost but it means you might have to weight some more lead.
I use a 10 pound steel plate with a STA and steel tanks at home because I dive 7mm, 14mm or drysuit. The buoyancy of my exposure wear means I want my gear to be heavy. When traveling, I want my gear to be lighter. Less weight in the luggage, I typically wear a 2.5mm wetsuit. So my travel BP/W is an aluminum plate and the wing has a built in tank stabilizer. Even if I need more weight, I'd rather use lead at the dive site than haul a STA and a 10 pound plate in my luggage.
Also, some wings have things like bungee cord to collapse it when you let the air out. Makes you more streamlined (or so they claim). My travel wing has bungee on the inside of the wing cover. This is supposed to be very streamlined. I didn't buy it for that reason. It was inexpensive but still well built. People tell me the bungee will make it hard/impossible to orally inflate the wing. If you cannot orally inflate a wing with bungee, you need to do more cardio.
There is probably more I'm not thinking of but this is what pops to mind.
Darrell
P.S. jut thought of one more thing... don't buy a wing which is too large. For single tank I use a 27 pound wing for cold water and 25 pound wing for warm water. I don't think I'd ever go above 32 pounds for a single tank. For aluminum cold water I might drop 5 pounds and for warm water I can probably drop another 3 or 4 pounds. This is my personal buoyancy characteristics. Yours will vary.
