Good choice.
I use a combro plate with a recwing, works very well.
As for the 3mm or 6mm, well the 6mm aleutian one will be heavier but to be honest you are a better man than I if you can bend or damage the combro plate.
I use a custom weight that sits in the grove of the backplate so I don't need any additional weight and thus the 3mm was fine.
As for the harness, mine is pretty minimal, no shoulder pads (Nope you won't need them), a few D rings, a bit of bungee for the LP corrugated hose and a bit of bike innertube on the other side for my mini Q40.
Finally as for getting into mine, I sit in front on the floor, put my arms back though arm holes and that's it, (well actually it is not quite as easy as it sounds but it works). Failing that you can extend one arm hole and use a single clip to take up the excess webbing, basically still one piece as on Dom's web site.
Dom's site
HTH
Daz