I don't think there is a minimum required for staking, someone correct me if I'm wrong. Pretty sure it just Stakes about 17% Of your balance in a little over a weeks time. Which is a pretty good investment, at any price.
You are right there really isn't a minimum to buy and stake however I will say this and that is the smaller amount of coins you are going to stake will take a lot longer to earn staking rewards for example I currently have a couple of smaller amounts of 7k and 10k and currently they are on 16 days with 40% chance to earn a stake in 24 hours. Your best bet is to buy a large amount if you can say 100k which is around $70 usd at the moment and you will most likely earn a stake in 10 to 12 days. This is just my suggestion and if you can't afford that much then buy a smaller amount but remember it will take longer to stake.