lets see, to get all the pokemon you need to trade between all the GBA version and Pokemon Colloseum. But you can't trade between Ruby, Sapphire and Colloseum until after you beat the Elite 4 and Champion, and you get the required items to allow trading. The only special pokemon I can think about being in FireRed and LeafGreen are the Gold and Silver, Crystal ? pokemon that if not found in the wild, are traded from Colloseum. If you don't know anyboy who has either version and you want them all then you will have to buy all versions, have two GBA's and the wireless adapters for FireRed and LeafGreen, and a link cable to link to Ruby and Sapphire, I don't think the Wireless adapters work with Ruby and Sapphire.
And whether or not it's worth starting playing now is up to you, really. I didn't play a Pokemon game until Ruby and Sapphire.