I have a couple things to add.
First, I suggest getting a current copy of the scale aerobatic rules, not just the flying and judging guide. You can find answers to your scale questions there. I believe the rules quoted earlier are out of date.

As for what plane to choose, there are just too many to name that would be great to get you started. I started with the Midwest Giles 202 w/ super tigre 2300. I flew this plane through basic and sportsman and did fairly well. This year, I've been flying a 73" H9 Cap 232 w/ Saito 180 in intermediate. I have to also recommend this engine! It's amazing! The H9 Edge would be great. The new Great Planes Patty Wagstaff 1/4 scale Arf is VERY nice as well. I flew a contest in Chicago last weekend where one of the guys was flying one of these w/ Moki 2.1. Excellent combo!
The most important thing would be to find a plane you're comfortable with, use the best servos YOU CAN AFFORD, and practice as much as possible!!
For your question about setting up your plane for IMAC, don't set your surface throws very high. Set your ailerons so you get about 360 degrees of roll per second max. Set your elevator so you can get nice smooth lines and still be able to pull out of a vertical dive comfortably. In other words, set your plane up to fly the sequences, not 3-D!! Work on flying as smoothly as possible.

Gary Hunt
Rantoul, Il