1:What makes baldurs gate so good (sorry for epic question) am i in for a treat as ive never played them before and know nothing about it.
Hard to explain really, it's just awesome

. Best explanation i can give is its the first game (and probably only) where i've felt like i've got complete control over my character, every quest has every outcome you could concieve, it's not 'good or evil', there's many subtle conclusions to every quest that you can achieve. The combat seems incredibly basic at first but once you give it a chance it becomes incredibly addictive. Remember, BG1+2 will provide literally hundreds of hours of entertainment, it puts most modern games to shame in that respect, but with that comes a much longer 'teething' period. You will not instantly love this game, but i implore you to really give it a chance as you will not regret it at all.
2:How long does it take to complete?
As stated above, hundreds of hours. The storyline itself, probably about 100 hours for both games if you rush. If you want to do all the side quests i would not be surprised if you ended up racking up 200-300. Some of the sidequests are just fantastic, the tower quest in the BG2 expansion alone took me around 10 hours iirc and the ending to that quest was something i'll always remember.
3: Im 99% sure about being a monk, i was one in NW 1 and 2 so i should be pretty good as a monk on BG, or is this a lot different?
Couldn't tell you if they're the same, but be warned, at low levels monks are quite weak, but once they level up they become godlike as their evasion is just stupidly high.
4: If im to be a monk what race is the best for a monk?
Can't help you there i'm afraid
Is the paladin quick to level up or anything? It's my first time playing this or any sort of major RPG so I don't want it to be too hard
They all level up at around the same pace. I just like the paladin as he's tough as nails, has access to what is arguably the best weapon in the game and has some medium level healing spells to give him that edge. They're just a nice, reliable, simple class to play as.