I'm really undecided on whether or not I should install Unfinished Business. I generally like to play games as they were released and untouched, plus I wonder if installing these restored quests would unbalance the game. (i.e. leveling too much and too fast early on, etc.) On the other hand, Unfinished Business restores content that was intended to be in the game but cut due to time and money constraints and I wonder if I would be missing out on some great story by not installing it.

Any suggestions one way or the other for a first time player?

The UB mod for PST is actually very minimalist. There's not an enormous amount of restored content, and the content that is there is pretty minor. Of course, if you enjoy games in their original state then there's no reason to install it even if it DID add some huge story revelation (which it doesn't). It's nice to have, but it's not mandatory.

I actually would almost recommend not installing the Fixpack the first time either, because it makes some content changes that I disagree with, such as altering how TNO's Thac0 is calculated, and making the boss of the Modron Cube's dialog missable. (If you don't interrogate him fully the first time you meet him, then you can NEVER speak to him again, which causes you to lose out on a lot of story and NPC interaction.)

In later playthroughs I've liked having the fixpack, tweak pack, and unfinished business, but for the first playthrough I didn't use any mods and had a perfectly good (and bug-free) experience. If you're planning on playing the game more than once (which you should) then you should consider making your first time mod-free as well.

Crap, I didn't know that and i've already installed the fix pack, though the main reason I installed it was to make sure I didn't have any quest stopping bugs or crashes as I know Black Isle/Obsidian are notorious for releasing some pretty buggy games.

One of the things I was interested in from the UB mod was Deionarra's Truth (non expanded version), that really sounds like it should have been in the original release from what I gather. Again, releasing games with cut content due to time and money is something else Black Isle/Obsidian are known for.

As for the tweak pack, Banter Acceleration and BG2 style ThacO display seem like something I should install, though I don't even really have a clue what ThacO is.

I think i've decided against the UB mod, but I think i'll install the accelerated banter part of the tweak pack as it would be a shame to miss out on some of the character banter and excellent voice cast. I think the one guy's mod guide recommended setting it to 30 minutes, any suggestions otherwise?

I have mine set to 30 minutes too. The banters are pretty short and superficial, though, mostly characters just randomly chatting about minor stuff. It's nowhere near the degree of BG2 party banter in that regard.

Of course, in PST you can actually talk to your party members at any time and there's an enormous amount of non-banter character dialog, so I don't consider it to be a shortcoming. Some of them are pretty fun, though, and the voice acting really is excellent. I couldn't look at Assistant Director Skinner the same way after playing PST, that's for sure.