Enrique, i have to agree with Adil. Not only is converting an existing airframe simpler and less costy, but also you have already gained experience on the airframe and know how it will behave. Only "unknown" remains the turbine characteristics.
By flying an airframe you already know, you do not have to worry about that and can concentrate on coping with the new powerplant. Definetly a good way to go in my opinion (thats also how i found my entry...)
regards