15K is tight with a DD of 9K and margin of 4K ,but you could trade with 15K , theoretically. I would recommend 20K-25K.
You can't realistically trade a 9k drawdown strategy with capital of 15k. A good rule of thumb is: your future drawdown will be 2x what you've seen in backtest -- and 18k would tap you out. Even 9k, equivalent to backtest, would cripple you -- you'd have 6k left.
Murray's system may well be very meritorious, but he doesn't help himself by recommending 15k capital. For a retail trader, 100k+ would be sensible; for an institutional manager (typically more risk averse), 200k+.

