That sounds similar to the system that Blackcoin uses.

If I remember correctly, those initial blocks are mined via proof of work in the same way Bitcoin is mined today. However, after a while, the coin disables PoW mining entirely and switches over to proof of stake. Hence there are two separate phases: a pure PoW phase and a pure PoS phase.

The PoW phase is there to distribute the coin. The problem of how to distribute a PoS coin is a tough one. Another option is to distribute them via an IPO where all of the coins that will ever exist are pre-generated all at once and sold in a massive auction. This method is popular but also somewhat controversial. Indeed this is the system that NXT, Qora, and a few other notable PoS coins have used.