I'm no expert either but I've read that larger bells equal more powerful motors. I know the QAV180 and 210 frame motor mount pattern will fit up to at least a 22mm motor. You could theoretically put a much larger motor on there as long as that motor has a mounting hole pattern that matches the frames pattern.
I started building a QAV180 about a month ago and went with the Lumenier RX1806-14 2300kv motors. They are light and pretty powerful and can handle 4s power with the right esc's. Could have went with a larger more powerful 22mm motor, but you can only swing a 4in prop on this frame...unless you radically redesign both motor height to allow for prop overlap and the location of the components mounted to the frame so the props don't hit them.
Thus the smaller and lighter 18mm motors to swing a 4in prop seemed to be the logical choice for me. Are you gonna buy the complete kit or start with the base plate? I opted to start with buying the 4mm base plate and purchased the super sweet aluminum standoff kit from the Flitetest store. I'm currently fabbing the top plate to mount the battery and mobius and the side plates to mount the board cam out of polycarbonate. I could go on and on about the current state of my build and what I've learned from trial and error so far but don't want to go too far off your post topic of motor choice.
When I went to mount the motors I realized the M2x5 mounting screws that came with the Lumenier RX1806-14 2300kv motors were not long enough. I then ordered a bunch of M2x8 screws and found they were too long hitting the motor windings and power leads. Luckily I had a big bag of M2x6 leftover from one of my micro builds that fit perfect. I would have ideally liked an M2x7 but used all four mounting holes with thread lock and feel that should be sufficient. I believe the kit has the 3mm base plate...thus the motor mounting screws is another thing to consider.
Check out this post about the QAV180 over at fpvlab. I was really excited and thought I'd be building one super badass QAV180, but after reading and seeing what some of the pilots there have built...I've been rethinking and redesigning my build ever since.