You'll need to buy the "skeleton" housing. This is the same as the waterproof housing, with the same two-prong connector underneath. But it's got cutouts at strategic places so that you can access the A/V in/out ports.
And obviously it's not waterproof...
http://gopro.com/camera-accessories/...eleton-housing
HERO3 Skeleton Housing
The aviator lens kit is essentially a Neutral Density filter. This removes a certain % of all visible light and that forces the GoPro to use a longer shutter speed to get the same exposure. The result of that is that the prop becomes blurred instead of distorted.
I have not tried the Nflightcam Aviator Lens Kit, but in general, results with an ND filter are quite good, provided you have the right filter for the light conditions. However, it seems to me you should be able to get sheets of ND filter material, so you can cut them to size and insert them between the GoPro and the housing, for far less than 80 dollars.