Thanks for the help Josh.
Stability will greatly help this project, so the lack of maneuverability is a bonus in my eyes.
So basically, get the endurance kit, and stick with the 4S battery?
The motors and the battery voltage must be matched. If you get the endurance kit,
linked here, you should use it with the 6S batteries it was designed for.
To go a little more into detail, a motor's kv rating is the number of rpm's per volt it will make, at no-load. So a 2000 kv motor supplied with 11.6 volts (nominal 3S voltage) will make 2000 * 11.6 volts = 23,200 rpm. In reality, once you put a prop on the motor, the rpms will drop, and the actual rpm made will depend on many factors. But comparing kv rating is a good way to compare motors. All else being equal, a higher kv rating motor will spin faster than a lower kv rating motor.
What about prop size? In short, a larger prop will require more power to spin at the same rpm as a smaller prop, because it encounters more air that it has to move. A larger prop will produce more thrust than a smaller prop, and more importantly, it will be more efficient. It is more efficient to swing a large prop slowly than it is to swing a small prop quickly. You will get more thrust for less power.
Finally, battery voltage. In short, higher voltage batteries let you select motors than lower voltage batteries.
The key takeaway is that all three components (motor, prop, battery voltage) must be well-matched. There is some variability, but only within a range. You can't take a motor/prop combo designed for 6S and drop them to 4S without taking a performance hit. Better to use a motor/prop combo designed for 4S. There are cases where it makes sense to mis-match your components and accept the performance hit. For example, the motors that I have on my copter now are able to handle 4S voltage with 10" props, but I'm only using 3S packs because I already have a bunch of them. If, later, I go up to 4S, I can expect longer flights, more power, and more efficiency. But if you're starting from scratch, do it right.