A Few Coins More
2nd Edition Advanced Dungeons and Dragons with minimal modifications. Game material will include the complete book series, but not the player option books.
Most likely training will be required to advance in level meaning returning to town and spending 100 GP and 1 week per level to be achieved (for each class if multi-class).
Training for new Proficiency slots is going to be allowed at 10 weeks and 500 GP per proficiency slot.
While we've done other advancement systems this campaign will likely be using normal XP advancement. Experience will be awarded when characters return to Greyhawk and players are allowed to divide the experience as they like between their classes, but can never gain enough in any class to skip a level (must stop short by 1 xp; if there is excess XP it is lost).
Characters get maximum hit points for class, and can go into the negative to their Constitution before dying (triple-class hit points are rounded down, but should be calculated at each level – F/M/T = 6 @ 1st, 13 @ 2nd, 20 @ 3rd, F/M/C = 7 @ 1st, 14 @ 2nd, 22 @ 3rd).