An M3, or 2 M2's strapped if your experienced with big power, keeping it clean, etc.
What motor is it? G, H, I? I just did a pair of 9518i's for a customer (I'm a DD dealer) on 2-M2's. They were 1st generation i models, with one less spider than the current ones, and not as stiff as the H generation than handled M3's all day. The i's handled the strapped M2's into clipping, so a clean M3 each would still be best.
With that budget, I'd get another 9518, and 2-m3's, just saying.