By the sound of it, ArduPilot is way more powerful and complex than what you're after. What you need is a simple feedback loop, taking the input from the horizon sensor, working out which way is up, and moving the ailerons to get 'up' back to where it should be. As far as I know, FMA Co-Pilot does that already. The ArduPilot system does that as justone small part of its operation, along with a whole load of other functions that fly the plane from place to place. If you don't want the other functions, then it comes down to a simple question: is the FMA Co-Pilot cheaper than an ArduPilot board, programming cable and XY/Z sensors? If the answer is yes, then just go with the FMA Co-Pilot.