I have not heard about having a minimum amount you must buy so far. Whatsoever amount you have will be converted to the amount of bitcoin what at the present point in time.
It's not about a minimum amount you have to buy but more a psychological thing: no matter how much
BTC is worth, at least you wan to have 1
BTC and not some mBTC. I remember back in 2011 when I started mining it took me some days to have earned my first
BTC – that was somewhat magical and it was "only" worth some 7 or 8 USD at that time. Don't underestimate the magic of having at least one coin rather then some mBTC.