It's not clear how much you've researched this, but to compete in pattern and IMAC will require two different planes. What's legal for one isn't legal for the other. Actually I guess you could fly an IMAC type plane in F3A pattern if it was within the size limits, but it wouldn't be competitive. But if the intent is simply to work on your aerobatic skills and you want a good plane to do that with, we can help you there.

I'm going to recommend one of the SPA (senior pattern association) legal airplanes. They are all in the size you're looking for, and they are a well-behaved, neutral handling planes that tolerate wind well and will have very little coupling with your control inputs. A few to consider (in no particular order) are the Tower Hobbies Kaos (easily the most produced plane in the history of pattern flying), the Goldberg Skylark (bigger and a bit slower), the World Models Intruder (crazy stable and smooth, but harder to spin), or a Dirty Birdy (great all around plane, but recently discontinued).

After flying with one of these old school pattern planes for a while, an IMAC plane will be a good third model. I'll second jetmech05's recommendation to go with at least a 30cc size model, but really you should get the biggest one you can afford. Bigger flies better, and it's easier to see. If you're serious about competition you'll have a hard time winning in a contest when other pilots are flying 40% models in the 200cc range if you are flying a 30cc size bird.