I'd guess 8 - 12kg, but there's far too many factors to have any confidence in that estimate.
I used to use 3mm gloves all year round and didn't have much problem... although now I use dry gloves which are far nicer, although more expensive initially. For wet gloves, you can get a kevlar coating which is good (harder to cut the glove on a wreck), titanium lining's i've never been convinced about, so that wouldn't influence my decision either way. Wet gloves with a velcro or similar wrist strap help reduce flushing (water moving in an out that makes them cold).
For a hood 5mm should be fine, I think that's what most of them are - you don't need the flexibility in your skull (!) so no point going any thinner really.
For a necklace I found the octoguard things (the bright yellow ones) kept floating away. I now use one made out of a bit of bungee and a few knots, which works far better, is far cheaper, and looks better! I used 2-3mm black bungee
You can probably find a picture of how to make one if you search through the forums.
One last top tip: don't try to open your mouth to talk underwater... doesn't work
David