Buying SP is wise if you want Steem to grow, as it's literally a long term investment, because you cannot take out your SP entirely for a minimum of 1 year.

This, I think, is not entirely correct.

1. You should buy SP if you have reason to believe that the system will grow, not if you WANT it to do so. Especially not if you consider to invest serious money. (Serious as in relative to your personal networth).
2. You can not take out your money entirely for a minimum of TWO YEARS (not one) - but: you can decide to receive your proportional weekly rates from day one.