Life span of a CPU ranges from 8-10 years. But I know nobody is going to use their CPU that long.
Once the lifespan of your CPU dies, then it dies. You get a new one and overclock it again. You don't have a need to change your motherboard unless you get a different socket CPU then the one that you had.
Both would be an okay option. Some people rather build their own computer since it will be nearly the same price as a pre-built computer and it has a range of choices for products.
Well don't worry about asking loads of questions, because someone will always explain it to you.
Overclocking really does shorten the lifespan of the CPU but only by 1-2 years. I mean, are you actually going to use your CPU to the full lifespan? I don't think so, because I wouldn't. Overclocking is really just to get a better performance out of the component. Makes your computer run faster if you do everything correctly, and it's pretty fun learning how to overclock and all.