Only invest an amount which you think you don't actually need for the time being. It is expected that you might need the money sometime in the future and so it is important to do it little by little as you have said. But as to the right amount there is no such thing as a standard for it. It is up to you. Diversifying may help and may not as well so it is a matter of choosing the right coin.
It may be an advantage or become a disadvantage for you if you diversify. If you have small capital then better focus in one coin that you think will have a better future compared to other coins. if you diversify too much then maybe you will only get small profit or other coins would just eat your profit because it has a big loss. Purpose of diversification is to lessen the risk so better have a strategy as well to cut loss if possible if worse comes to worst.