So, I have not yet purchased BG2, but I do plan on doing so. I only have a few questions, and to make this easier, I'll number them. If you know the answer to a question, put the corresponding number next to your answer.
1. Do I need a CD key or does it come with one?
2. Will it work just fine natively with Windows 7. I tried to use my original disc's for BG1 and had sooo much trouble that I just gave up.
3. Can multiplayer be done over the internet?
4. If I buy this game, can I let my brother use my copy too? If so, can we play multiplayer with the same copy?
5. I have played this game once, it was a very long time ago, I'd like to enjoy it as it was, are there any packs/mods that I NEED to play with, or any mods that ONLY make it better. I do not want content packs (unless of course it includes more vintage-like faces) [Like the Morrowind high res mod that strictly makes the game look better]
For 1-3, Hickory is correct.
For 4, you can, but whether you should
is up to you. Being DRM-free you can do whatever you want with any game you buy here, including copying it 10 times and giving a copy to each of your friends.
Just remember that if everyone did that, GOG would likely go out of business.
For 5, you don't really need
any mods, but the BG2 Fixpack is strongly recommended. It gets rid of almost all the bugs and makes playthroughs less frustrating.
For convenience, you might want to try the BG2 Tweakpack, it doesn't affect game content, but it can be used to change some things you find annoying (mostly making inventory management much easier). It's also modualr, so you can choose to only install the tweaks you want.
For graphics, you might want things like the widescreen mod that lets you play at higher resolutions or 1pp, which gives you better-looking icons and models for some things.
I strongly disagree with Hickory about Ascencion and Turnabout. Not only do those mods change a lot of content in ToB, it also makes that part of the game noticeably harder.
Even Unfinished Business only adds content, even if it's just things that were cut out of the final game by the devs.