It is not required to purchase a whole amount of bitcoin. You can opt to purchase a hundredth of a bitcoin or even less. With regards to keeping your bitcoin in coinbase, i have a few buddies of mine that is keeping around 3-4 bitcoins in Coinbase, so I would say that there is no problem with their security since my friends were able to withdraw his funds anytime they want. Nevertheless, keep your funds scattered, don't put them all in one place would be the best option.