I don't think he's looking to fly military fighters. Nobody sustains 7-9G in any piston plane, aerobatics or not. The Unlimited monoplanes might pull G is this range, but it'll only last about 1 second. There's no need to pull more than about 4.5G at the Sportsman level. And even at that, the G is experienced for very short amounts of time. You really don't have to be in great shape to fly aerobatics, or even competition acro. If you came to a contest you might be surprised that many guys have the same beer guts that non-acro guys have. :-)
I would recommend an RV. They are designed and stressed for aerobatics (+6/-3G) just like any other aerobatic airplane. My Pitts has the same stress limits. They are perfectly suitable for flying Primary and Sportsman level competition acro. Any airplane has its limitations. You would not subject an RV to the same stress and certain maneuvers that are possible in the carbon fiber monoplanes, but neither would you most other aerobatic airplanes that are not carbon monoplanes. I flew aerobatics for 5 years in an RV, and did all the aerobatics you can do in an airplane without inverted systems. Great airplane.