What are the best starting character specifics? I would like to play the ones for the most immersive and interesting playthrough (note - I like to resort to pacifism and dialogue-options in this kind of games, resorting to combat only when the situations demands so).

I learn that some of them can be improved, just like in Fallout, but a lot more. Can anyone elaborate a bit on this, give some recommendations?

I'd recommend 15 INT, 16 WIS, 13 DEX, then putting the rest into CHA. When you level up, you get a statistic point to allocate in one of your stats (unless you have started in another class, in which case you don't gain new stat points until your new class has more levels than your old one; yeah, old D&D rules for mixing classes were weird, and the system PST uses is very much so in that regard). With those stats, you might want to look into becoming a mage, but that's just my advice; if that floats your boat, then try finding someone in the Hive who can teach you magic (I can spoil and provide more data if you want). High wisdom and intelligence are needed for certain dialogue options, and charisma and dexterity allow for certain alternate resolutions in some scenarios. Just make sure to pump your INT and WIS to about 18 or so, then maybe look into buying some stat-enhancing gear (there is a tattoo shop in particular you should check out)

I really think that if that is what you are going for, there is no better starting setup than 18 INT 18 WIS 12CHarisma. Then get to Mebbeth as fast as you can to become a mage as soon as possible.

Well how is as fast as I can? I've played already once and it was actually quite late after a long time that I even saw that I could switch into a mage. She's in some other part of the Hive, right?

As for 18 for WIS and INT, isn't that a bit too much? If they can be further enhanced in the game, wouldn't maxing them at start seem a tad wasteful?

Well, 18 isn't the maximum, just the maximum for character creation (ie the maximum that a normal person could possibly achieve). The cap is actually 21. If you started with 18 Wis, you would end up wasting some stat points, but only right at the end of the game. Moreover, a higher wisdom means you get more xp, which means you level up more and gain more stat points in the long run.

On the other hand, if you want to have all the dialogue options, I'd recommend reducing the wis, and int in particular, by a point or so, and putting the freed points into cha. As far as I know, there's no need for a massive wisdom/intelligence in the early game, while improving your charisma a bit will open more dialogue options. There are also a few dialogues involving dexterity, so you might want to improve that a bit too. Unfortunately I don't know how much dex you actually need, since in my playthrough I left it at minimum and failed the checks!

I'd suggest this lineup:
9 Str
12 Dex
9 Con
15 Int
16 Wis
14 Cha

And then I'd put points into Int, Wis and Dex for your first few levels.

You don't need to reach Ravel to become a mage. The person who teaches you is indeed somewhere in the Hive, and if you leave the Hive before you find him/her then you should probably go back and talk to people a bit more. It's OK to wait a little before becoming a mage; because the xp tables are exponential the xp you'll lose out on isn't that important (and you'll end up with more hp in the long run, too!). The early fights shouldn't be too difficult, since the designers realise that players may have low physical stats at that point anyway. From a metagame standpoint I'd advise becoming a mage before you hit level 7, since you get a bonus according to which class hits level 7 first. For a mage it's a stat bonus to your mental stats; for a fighter it's help with your weapon skills. Guess which is more useful to a mage.

SPOILER:

Well, technically you do need to meet Ravel to become a mage, but only because Mebbeth is Ravel in disguise, something you don't find out until much later.

Having fiddled around with the stats early on, I think that in order to make all those checks you need a 13 in DEX. Granted, with some mods, the game actually makes you roll for them, so even that might not guarantee success if OP is using mods. High Int and Wisdom are actually very helpful early on when it comes to interactions with Dak'kon.

Also, isn't the cap for stats 25?

Fair enough, and yes, you're right about the cap. I must have been getting muddled with something else, maybe the fact that my Baldur's Gate character had 21 wisdom for most of BG2.

Ah, gotcha. But yeah, in AD&D, 25 is where the devs stopped statting things because anything with stats higher than 25 was pretty much a god at that point.

So in short, when I get out of the grave, the first thing I have to do is enlist that warrior-character (skipping everything in the first screen), and then do as little quests as possible before going to that other party of the Hive and switching to a mage?

That's basically what I do. Finish the entire mortuary first of course, then once in The Hive I make my rounds and pick up all the quests I can as I make my way over to Mebbeth's hut (but without going out of the way to get them.) I wouldn't necessarily skip the first screen (the northeast hive), since you're there, do whatever you want really! I don't enter into any of the buildings, I just do the outside bits, screen by screen. She's a few screens over in the hive and there is a lot you CAN do before you go to her, but you can just bee-line straight for her too. I just do the most basic of things on the screens between the northeast and ragpickers square, because once you get to her she gives you one quest at a time to go around the hive and you can pick up all of the other quests while doing those errands. I complete all of her quests and then switch to a mage and then continue on around The Hive and finish all the other quests I can/pick up the rest and then go in the buildings. And yes, you can get Dak'kon before getting to her.

Personally, I ended up going all the way to early level 6 before finding how to switch to a mage. It didn't present any problems, since the xp needed to gain a (for example) mage level is based solely on how many mage levels you have, and not on how many fighter levels you've got. The xp requirements also grow rapidly, so the amount of xp which goes into fighter levels rapidly becomes negligible, and of course you get extra benefits like HP and thac0 from any fighter levels you gain. In fact, even under normal 2e dnd fighter>mage dual classes tend to be more powerful than normal mages. Just don't go above a level 6 fighter if you want to maximise your stats, as you'll miss out on the mage specialisation bonus.

Not exactly; you can become a mage whenever. The only downside to waiting too long is that you miss out on some stat bonuses at certain levels. Even then, doing most quests in the Hive will still leave you plenty of room without foregoing those bonuses, if RP is a concern.

According to http://www.gamefaqs.com/pc/187975-planescape-torment/faqs/7964 there's also a possible experience bonus from Ravel if you previously got the mage specialisation bonuses. Experience can be gained in other ways, of course, so it's not that important.

Low STR and CON can be a hindrance early on when your best options available are for warriors. You can start dumping lots of points into weapon skills early on, spells are scarce and most of the ones you do get are weak offensive spells or weenie buffs.

With a low STR you won't be able to hit squat and your damage will be feeble and with a low CON you will regen health at an agonizingly slow rate. Having deficient DEX is also a bother. Avoiding damage entirely with high AC is certainly useful and there are a lot of handy checks that can only be passed with a DEX DC of 13 I believe.

There are opportunities to raise WIS by completing some quests and making some checks, so leaving a few points off WIS in order to get your WIS up to at least 15 by completing some quests early on will allow you to service some of the warrior abilities at character creation. I recommend this.

If you think you can use companions as front liners remember they do not regen health over time like Nameless. Health restoratives are weak scarce and the useful ones are expensive early on, so damaged allies can easily become a black hole for bandages/needles/clots etc unless you bother to cross the map to rest every time your party is bruised.