If it already has a 3cm hole then it sounds pretty bad. Like others have said, best to get a porfessional to have a look and give you a quote if you don't know how to do it yourself.
Anything is fixable, but is it worth getting 5 or 6k worth of rust repairs done to your car when chances are that you could find another celica with less rust for 3k? I think not
I'm in the same boat at the moment with my car. I could probably pick up a new celica for the less then what it would get the rust removed profeesionally from my car, but with mates rates, and the car being my dads straight form toyota, it's a better deal for me to fix the rust, and have the car I wanted since I was kid.
Weigh up the options and see if it really is cheaper to fix the rust or send your car to the wreckers.
As for engineers knowning about bodywork, I'm a third year mechanical engineering student and I know nothing about bodywork couldn't fix the rust in my car anywhere near as easy as a pro