i've seen hacks for 4-in-1's that make them work with brushless motors. using the original Tx,Rx, Gyro, and mixer, the only costs are the wires, motors, and escs (probably $150 to $200 total for the lama). you need some decent soldering skill as you'd have to open your 4-in-1 and solder esc power and signal wires to it, or you might find someone in these forums who can do the mod for you for a small fee.

i'm not sure on which particular 180motors/gearing work best with a lama, but believe it's been done before and is well documented at rcg. you could surpass 20min flight time with it or carry just heavier loads.