Wings, I use 15% only because it burns better in my 4 strokes and I like having only one type of fuel. For a 2 stroke, 5 - 10 % is plenty.

As far as 4-strokes go, don't believe any of the hype about their maintainance. I have mostly 4-strokes, and they ae no more difficult to run or maintain that a 2-stroke. And you DON'T need a tach to get them to run right.

If you WERE going to get a 4-stroke, I would recommend getting something in the 70 size range. While a 52 would be plenty for your Spacewalker, you'll find that a 70 will give you a much larger selection of planes to move up to in the future.