Quick question about brakes

So, I've been looking into getting my girlfriend some disc brakes because the ones she’s currently running are absolute garbage. Everyone keeps raving about the Shimano SLX M666 as the ultimate budget-friendly powerhouse. I spotted them over at but from what I can tell, am I getting everything except the rotors? Also, which rotors should I actually pick up? There isn't any info here regarding what size will fit.

You might be overspending here..
2 x $100 plus rotors and adapters comes to $667

EDIT: I just read that at Bike Mania, which is also a similar US-based shop, you can grab the whole kit with XT 180mm rotors for about $1,500 - $500.

What kind of bike are you planning to put this on? Is it actually designed for disc brakes? And what makes you jump straight to Shimano SLX?
By the way, if I were you, I'd probably just service the brakes you already have. If they aren't stopping you properly, something's definitely off—you likely just need some fresh pads, a good bleed, or maybe just some lube on the cables. A little TLC usually does the trick.

You can opt for the Shimano Deore M596 without any lingering guilt; the cost is significantly lower, and frankly, if she has been struggling with bottom-tier brakes until now, she won't even notice the difference.