hey neowarriors, welcome.
here is a link to a site that has a ton of info on the fluff of the warhammer universe: lexicanum
as for the life span of a space marine, I was under the impression that they were technically immortal, as they will never die from old age, only die in battle. As for life-span, one of the oldest space marines still in service (not counting dreadnoughts) is commander dante of the blood angels, and he is going on 1100 years.
the gene seed is basically the DNA pattern of the space marine, modelled after the emperor and the primarch. apart from the extra organs and implants, this is what makes a space marine more than human.
when creating a chapter, if you are going to use it in official play, you won't be able to make up any of your own rules, but you can make up any backstory for them.
the best thing to do is to find the chapter that most closely resembles your creation and use those rules. You can even use the same characters, just change their names and they are good to go.