If you're serious about the game, I suggest getting ahold of the Dungeons and Dragons material that it's based on.

Player's Handbook
Dungeon Master's Guide
Monster Manual

These are the three core rulebooks, and they're basically an absolute must as they define the stats for classes, weapons, armor, monsters, etc. etc. etc. that the games are based on. You can pick up the complete set for $60 or so. Alternatively, if you are dead-set on re-using the older rules as opposed to the newer 3.5 rules, you should get your hands on those earlier rulebooks.

Pick up the Forgotten Realms Campaign Setting book. It contains summaries, timelines, statistics, and important people and places for the entire world in which the Baldur's Gate games take place.

At the very least anyone seriously putting together a fan-made BGIII needs the core rulebooks and the FRCS. If you want to make it good you need substantially more, including more specific books about Faerun's regions, advanced monsters, gods and avatars, etc. If you want to do some planar exploration or use characters from other planes you'll want the planar handbooks, and possibly a deal of information from the Planescape setting.

I'd also suggest reading a number of Forgotten Realms novels for a look at the people and places involved in the Baldur's Gate games. The Avatar cycle: Shadowdale, Tantras, Waterdeep, and the two later ones about Cyric are important to read because they tell the story of the Time of Troubles including detailing Bhaal's death and the people and places involved. Many other FR novels give looks in at Baldur's Gate, Waterdeep, Amn, Calimshan, etc.

Without doing the research beyond just playing the Baldur's Gate (or the few other Bioware) games you might as well not start. Without staying true to the setting, even if the game contains many immensely cool aspects, you'll get wide unacceptance from people who take the BG games and the FR world as "living" storyworlds and not just games.

At any rate, putting together a good story and doing art and music are going to be your biggest challenges. Once you find them a few skilled computer science majors with some spare time, the appropriate software and hardware, and an interest in programming games can build you a game engine IF you can tell them exactly what you want to accomplish (the rulebooks are invaluable here, as they basically lay out how every stat can be incorporated into most typical player actions). I'm not sure, but you may even be able to use the Infinity Engine that BG and BG2 used if you can get ahold of the myriad of editing/hacking programs you need. Alternatively, you may just want to mod NWN2 if it is versatile enough to do most of what you want to accomplish.

I also recommend going around to other fan-made game projects on the net and gathering information on how they approached their projects, where they encountered difficulty, what file formats they used, etc. Hero6 is one I spent some time with myself many years back, but there are many others out there.

Finally, anyone can think up some ideas for a story, lay out some characters, sidequests, etc. This is done by thousands of people every day. If you're serious, you really need to take it upon yourself, or you and one or two other people, to be a Project Manager/Director. Determine your vision for every aspect of the game, right down to storyboarding the movie sequences, and then go find the people that can make it happen. Letting a bunch of people into every aspect, or too many aspects, of the decision-making process assures that your game will take the better half of a decade to get made.

If this sounds like a full-time job, that's because it is. An rpg of playable quality is not something that people generally accomplish in their free time...that's why people get paid to do it. This is doubly true for a game like those in the Baldur's Gate series...to maintain anything close to the level of quality you have to maintain the amount of work and information gathering that Bioware/Black Isle put into the games when they made them.

Anyway, I don't mean to discourage you. I'd rather somebody with serious interest know what they're getting into. People with real experience (that doesn't mean scribbling your plot, some character info, and a few game features on the back of a napkin) in making games, particularly true gamers, can only mean better games for all of us in the future, so I wholeheartedly encourage it.

As a final suggestion:

Lay out the entire framework of your story.

Who is the main character?
Who is the main villain?
What is the villain's motive?
What is the main character's motive to stop him?
What locations (in the broad sense) will the player be traveling to?
At each point of the story, what drives the plot forward?

Top down rather than bottom up. Take some time off from the classes/races and related sidequests and develop a cohesive story. Then go back and fill in.

Here's the example from BG1. Until you have the story laid out and "set in stone" you don't really have anything but "some scribbles on the back of a napkin."

Baldur's Gate SPOILERS:

Main character: Child of Bhaal, the Lord of Murder

Main villain: Sarevok, another child of Bhaal, by a different mother

Villain's motive: Sarevok wishes to claim his "birthright" and become the new Lord of Murder. He hopes to accomplish this by causing death on a large a scale as possible.

Main character's motive: Discovering his heritage, avenging the death of his foster father, and in the process foiling the plot to bring war and chaos to the region.

Locations: Candlekeep - your home and starting point; Beregost - the town caught in the middle of a potential conflict between the great region of Amn and the city of Baldur's Gate; Nashkel - frontier mining town under Amnish control, source of bad iron plaguing the BG regioin; Bandit Camp - hidden camp of the bandits destroying commerce in the region; The Cloakwood - large forest with a secret mining project working for the benefit of the Iron Throne; Baldur's Gate - seat of government for the region, under control of "the Dukes", kept secure by the Flaming Fist mercenary security force, Iron Throne trying to grab power by being sole provider of good iron to the city.

Plot points: Forced to leave Candlekeep by your foster father Gorion-> Gorion's murder leads you to seek help from his friends->iron shortage/plague and bandits causing unrest in the region and his friends want you to investigate->discovering source of the iron problems leads you to conclude that the bandits and the iron problems are being directed behind the scenes by somebody with a larger plan->investigation of the bandit camp leads you to The Cloakwood where the Iron Throne is mining good iron to sell for their own profit->Iron Throne is trying to consolidate power in the city of Baldur's Gate and the bandits and iron shortage were part of that plan->you are framed when you infiltrate the Iron Throne meeting at Candlekeep after the heads of the organization are slaughtered->Sarevok operated behind the scenes to use the Iron Throne to grab power in the city and launch a war on Amn->Sarevok is your half-brother and wants to cause war and chaos to ascend to divinity.