every time i play baldurs gate i never finish it,
and i plan to start playing again, and i plan on finishing it this time,
so what in your opinion is the best class and best race to start with?

1. Baldur's Gate 1 or 2?
2. Vanilla or TuTu/BGT?
3. Power Gamer or Role Player?

I suspect I know the answers to 1, 2 and 3, but equally I could be wrong; it's important to know.

Less important -- many people have a preference -- but 'best' class and race can be built around preference:
4. Do you have a personal preference to race?
5. Do you have a personal preference to class?

A fairly standard build is to play through BG1 with a human Fighter up to about lvl 8, import your character into BG2, then duo them into a Magic-user at the start of BG2.

So you woud want high Str, Dex, Con and Int to do that.

In BG1 dual classing is pretty pointless, so you're best starting with a single class. Now as to which is 'best' will depend on how you intend to go through the game. Since you say you are likely to role play to some extent, mixed with power gaming, I will assume you'll be taking a balanced party with you. That said, your first party member is probably the best NPC in the entire series, stat-wise... Imoen. That means you have a superb thief/scout -- she can't carry squat due to her low strength, but that doesn't matter, since you just give her all of the light stuff (gems etc.). If you intend to do what a lot of people new to the game do, and go with the 'storyline' team of Imoen, Jaheira, Khalid, Minsc and Dynaheir, you will have your warriors, thief, mage and a dual druid. That means your best main character is unquestionably a Cleric, since you won't have one otherwise. A Cleric is pretty indispensable, especially for new players -- you will have access to the best cleric weapon in the game very early on (+2 hammer).

If I'm off the mark, and you intend to play a different squad, or not one at all, let me know.

I'm going through vanilla as a fighter, does this mean I need to dispense with one of the "storyline" team of Imoen, Jaheira, Khalid, Minsc and Dynaheir for an NPC cleric? Is there an NPC cleric in the game, and if so, what team member should I replace them with?

You don't have to, but your team will benefit greatly from a cleric. You should be aware that getting rid of anybody in your team except Imoen will cause the 'other half' to also leave: if you dismiss Jaheira, Khalid will leave; if you dismiss Khalid, Jaheira will leave; if you dismiss Minsc, Dynaheir will leave; if you dismiss Dynaheir, Minsc will leave. This is true unless you are unscrupulous and cause one half of the 'team' to come to an unfortunate end, so they are removed from the party, or you can dismiss the dead member -- not a good idea if you are role playing.

There are 2 full clerics in the game: Viconia (evil) and Branwen (neutral), and both can be recruited reasonably early.

If you do decide to dismiss one person for a cleric, do NOT dismiss Imoen, as she's your only thief, and she is essential for disarming traps -- no other class can do this. Probably the best person to dismiss (to cause to have an 'accident') would be Khalid, since your character is also a pure fighter.

Here is what DSimpson says about those NPC teams in his walkthrough found at www.gamefaqs.com:
"To get one of these in the pair without the other, simply send the other person into a building (the person you want to get rid of), and then reform the party to kick them out. This way they can't come up to you and talk to you."

When I played through, I changed the composition of my party according to what I needed to do. It requires a bit of grinding to level-up the newer members, but it isn't that bad if you know where (and some members will join with higher level). There is an experience cap in vanilla BG1 anyways, so after a point you won't be able to take your characters any higher, so you might as well switch.

A common way to play Imoen is to dual-class her into a Mage after about lvl 6. THis is how her character starts out in BG2: a dual-class thief/mage.

So, for example, if you are playing as a Fighter and once Imoen has dualled into a mage, then the Minsc/Dynaheir team becomes sort of redundant. Both Viconia and Branwen make excellent meat-shields as they can wear any type armor plus have all their defensive priest spells. And imo Khalid/Jaheira are nice to have, but neither is absolutely indispensable to a viable party.

Also, one advantage of multi-class over dual-class comes in BG2: you can gain powerful epic abilities, but only in a class in which you can actively level-up. Thus a dual-class fighter/mage won't ever gain any epic fighter abilities in BG2.

One more thing: if you play through BG1 in its totality, you will find potions that boosts your character abilities, plus some magic weapons that boost them even further. This makes certain power-gaming character builds viable. For example, start with a dwarf as your PC having 19 Con. As you explore you will be able to raise it to 20, and your dwarf will regenerate HP automatically each round. Thus taking a dual-class dwarven fighter/cleric can be pretty bada$$. IIRC this character build can level-up to a 7/8 fighter/cleric under the vanilla exp cap (exp needed to level for both fighters and clerics is pretty low).

And lose your thieving abilities for the party for more than half of the game? This is not common, and only viable if you have another thief in the party. With only 8 CON, Viconia makes a TERRIBLE meat shield. Regeneration from high CON does not restore HP every round, at least not to any meaningful degree. It takes so long that only during resting, or traveling will you notice any difference -- it is NOT like a ring of regeneration that restores HP every 6 seconds. Nope. The highest possible fully dualed Fighter/Cleric is 6/7 -- a 9/9 fighter cleric would not only have the fighter class still inactive, but would need 475,000 XP, when the cap is 161,000.

There are tons of thief NPCs in BG1 (Alora, Coran, Skie, Safana, etc), thus losing Imoen's abilities for a while is no problem. Arguably you mostly only really need them in Durags Tower anyways, so just stay out of the tower while leveling Imoen as a mage, or go there before you dual her.

Regeneration rate for 20 Con is 1 HP/round (60 game-seconds or 2.5 real-time seconds). It will make a difference if you buff properly.

re the exp cap: yes, you are right, I edited my post, but you ninja-ed me first.

The OP will be role playing, at least to some degree, so switching party members willy-nilly would be a non-option. Not to mention that Alora and Skie can only be found very late in the game.

No, it is not. The rate is 1 HP per TURN. That is 1 HP per 10 rounds.

i will not switch party member, except for khalid and jhahira

You will miss out on a lot of the fun potential interaction between the different NPCs if you don't switch out. But to each his own. I think it is more fun (and just as serious roleplay, especially with a neutralish/chaotic character) to switch.

There is no party interaction in vanilla BG1, unless you count fights that might break out between 'incompatibles', or the occasional non-interactive comment.