The kit for the Senior Kadet is not all that expensive and it is a great airplane. I've built three and enjoyed them all. The ease of having a box open and finding all the parts you will need inside will make the build more enjoyable. This is a very easy building type and flies so well you will enjoy every minute you have it in the air.
The power you put in it will depend on the altitude ASL of your runway. A .40 wouldn't handle a Senior for me. Our runway is 5,000'
ASL so I've had .60s in all of them which makes the take off much easier. Once airborne 1/2 throttle is usually enough. If you are at 1500' or less a .46 would be a perfect engine.