I know when you buy your first gun, mountain bike, dive gear, etc. It tends to be like this is the only one I'll ever get, so I need to make the right decision....
But if you stay in the sport its usually the first of many,....
Good you are doing your research,
But don't get to hung up, first good choice is a steel backplate,, that will last you for life, (but I don't know how many backplates I have now)
Harness is replaceable and configurable,
( i am still changing mine and reconfiguring after 20ish years of diving).
Wing is where you are more than likely to change or get different sizes,
(I got a 60lb DiveRight classic was to big for what I needed at first, but I am very happy with it.. 18ish years and still using it..)
if you plan to dive single tank get a smaller wing, or PM me and I'll show you what I did to a larger wing,
When you want to go doubles either get a bigger new wing or you'll know someone that's selling a used one etc,,,,,
Oh
If you are long torso make sure you get a long plate,
makes a huge difference having the waist strap come off closer to your hips,
A real pain when its higher up...