maybe he doesnt read the op's question i guess. buying bitcoin in $100 amount would be small to keep it for a long time, with that amount i would rather invest it into trading and do my best to make it grow with it just avoid on lossing trade it cant be possible but be sure to do smart trades so that your $100 in bitcoin would grow not just on holding it.
$100 is not really a small amount for everyone but it is small if someone is wishing to hold his coins so for that he would need to grow them a bit by trading altcoins which can generate him good profits gradually even if he starts with $100.