voodoo47: the game has no serial key, so no serial key can be obtained from anywhere. you should still be able to add the game to steam as a non-steam game, if you already own it.
1) no. Valve knows jack about System Shock - any future updates will come from systemshock.org, and will have to be installed by the user. and actually, the steam auto update system will probably interfere with this, unless disabled.
2) whatever
3) steam overlay works for all DX9 games, and therefore should also work for SS2 (unless you disable the DX9 renderer).
one thing you should always be aware of - with steam, you are adding one more layer of 3rdparty software that wasn't originally there, and that will
always increase the risk of potential issues, no matter how much argument.
And a good reason for GOG to not give out Steam keys is that GOG would probably have to pay money to let you play their game on their competitor's service.