Becmi/Od&d is my preferred ruleset, but you can build your Pc with any ruleset you are more familiar with. That's because PCs will begin with just characteristics and some abilities. It's not that rules will not exist, it's just they will be integrated into the story, not the opposite.
Example: Gravesgard plays a gnomish bard. He will start at 0 lvl with only some basic proficiencies or abilities of his class and some skills, but soon he will gain levels and powers.
Powers here is a generic term for spells, feats or class specific abilities and proficiencies. You can mine any D&D book of any edition for your powers, but you still have to find a teacher to learn them. If you do not want to search them, I'll propose some to you.
Any power will be then translated in-game by me as appropriate for the game world.
I will explain all this better when we get to it.
So Gravesguard will not be exactly be a gnomish bard, but a gnomish music master, as a bard, in Karameikos gnomish society, is also a builder of musical instruments, not just a player. If he is human, he could be a thyatian musico (musician), a traladaran folk singer or else.
Dervish please reply to the questions in the other thread too!