Turbine Size
The AV8R was designed for one of the 12 pound thrust engines such as a RAM 500 and so on. The bigger (and heavier) engines would cause you to need considerable nose weight. The thrust plume from one of the bigger engines is wider which would begin to interfere with the airframe necessitating the need to move the engine further to the rear to clear the airframe. This would in turn only demand greater nose weight leaving the model overweight which would ruin some of the characteristics for which the model is designed. For this reason I would suggest you not install the Mercury on AV8R.
Gerhard,
I currently have no European distributers for AV8R but would be happy to sell you one direct.