Since you are building from a kit a light powerful motor might be more fun than a heavy Ball Bearing motor. I had a fox 40 bushing on mine. The 4*40 I had was built very light. The guy that built it installed a new fox 40 bushing and flew it several years. He sold it to the guy I bought it from who flew it 2-3 years. I bought the plane(still had the fox 40) and flew it around 5 years. The covering started coming off and I sold it to a newbie for a second plane(I still miss it). He recovered it and is having a blast with it. The Fox 40 has way more than enough power. It is very light and It will probably last forever. Sheldon's sells them for 71$.

http://www.sheldonshobbies.com/listing/fox.html

Edit: Wow seems they haave a sale right now! 54.99$ for the Fox 40 bushing engine.
http://www.sheldonshobbies.com/listing/airhd01.html