Are you playing the first or second game, or both? If both, I highly recommend either Tutu or the Baldur's Gate Trilogy mod. This allows you to play the first with the much better engine improvements of the second game.

As for Unfinished Business, I'd say yes, go for it. It won't ruin your first experience at all. Did you check the stickies for ideas for other mods?

The mods I used for my first playthrough (being that I had already run BG1 many times and knew the difficulty) included:

Unfinished Business
Xan NPC
Ascension

BG2 Ascension is an amazing mod (and semi-official, too) - adds more difficulty to the ToB bosses, gives extended epilogues and has a bunch of extra dialogue.

Unfinished Business is pretty good to have - it doesn't add anything major, but the bits it does add are nice.

You should look at downloading the BG2 Fixpack from the Gibberlings website. It fixes hundreds of little-known bugs in the game for a smoother experience.

Are you playing vanilla BG, Tutu/Easy Tutu or BGT?

If vanilla: BG1 Fixpack, Unfinished Business, Xan Friendship, and BG1 Tweaks (though these should be limited for a first time run).

If Tutu or BGT: BG2 Fixpack, BG2 Unfinished Business, Xan NPC, and BG2 Tweaks (again these should be limited). I'd avoid Ascension, however, because of the tactical changes it makes. It's more geared for the experience player.

Unfinished Business and Xan are hosted at the Pocketplane Group website.

The Fixpacks and Tweak packs are hosted at the Gibberlings 3 website.

I only own BG2 so this is about that, sorry for not specifying.

Also, I was thinking about getting the Sword Coast Stratagems II mod, because it looks appealing. Is that a bad idea for a first playthrough?

And does it conflict with the other mods (unfinished business, tweaks, etc.)

thanks guys

In my experience, no, it probably is not ideal for first timers. You want to learn how the game works before you makes things difficult on yourself, and (similar to Ascension) it might end up frustrating if you're not used to how combat normally plays out. For example, you absolutely need to know how all the anti-magic counter-spells work, and it makes some early battles nearly impossible on account of not having any high level spells to take down enemy protections. I played it my first time through and ended up having to remove some components. (Now I'm further on, I do enjoy the extra challenge however)

None of these mods really conflict as long as you install in the recommended order (see the sticky threads and mod readmes)

I agree with anamorphic completely. Most mods now use WeiDU as the modding tool and eliminates most mod conflicts. BG2 Fixpack, BG2 UB, BG2 Tweaks, all should have no problem working together, and any known problems should be noted in the readme's.

As for SCS II, look over the components and avoid those that increase or change combat. I'm not that familiar with the mod, but I believe there may be a component or two you could use.

I recommend making your first playthrough totally unmodded, not even using a fixpack to get the original experience.
Mods increasing the combat difficulty should be treated with care since they'll make you get used to exploit cheesy methods you'd rather not like to know and destroy the last bit of roleplaying illusion.

I would say for first time just use widescreen and bg2fixback. I don't see how the fix pack can alter the original experience. They are very careful in what they include. Also highly recommended is chosen few components of bg2tweaks, such as the ammo stack increase, to make inventory management less of a chore.

Unfinished business is mostly ok, although as someone else said there are certain restored content that makes you think they were right to cut it in the first place. Not bad, just not really worthy.

I would definitely discourage from using friendship and npc mods. Better to stick to official characters and dialog, to be able to decide later on if third party content blends in with the game or not. Kinda hard to do that if you don't know what's what.