Hi Davy,
Skill level is based on many factors and is also in the eye of the interpreter. Confidence and ability to fly the aircraft one to two steps ahead would be deemed as a competent flier but then every aircraft has different flying characteristics and a slow flying forgiving trainer and a fast aerodynamic Mustang are at different ends of the skill level.
Confidence and skill level would be how comfortable you are at correcting the aircraft's direction or sudden movement from the transmitters control sticks in an automated action without having to think about what you have to do.ie you fly the plane instead of it flying you. When the plane is flying you, you are one step behind and always correcting its moves instead of making it follow your moves or intentions.
Now skill level on one aircraft does not transfer to another aircraft. Every plane, even two of the same type and size will have completely different flying characteristics but will have similar operation of takeoff and landing speeds.
Only you will know when you are ready and confident enough to fly your Mustang and will always be a nerve racking experience, but that's the thrill of flying model aircraft. ( The adrenalin Rush ).
I do however highly recommend that you have an experienced pilot fly and trim your mustang and sort out any odd tendencies it may have before you attempt to fly it.
Also remember that unlike small electrics and trainers your larger heavier mustang will require a much higher landing speed and higher stall speed which changes landing approach speeds and angles, hence different SKILL LEVEL.
Even very experienced pilots have nerves and shakes when testing or flying a new aircraft but ultimately Davy,Re only you and you alone will know when you are ready to fly that mustang and DO NOT let anyone talk you into flying it if you are not ready to do so. IT will ultimately lead to its demise.
Even get someone else to takeoff and land it and you fly three or four mistake high until you are confident in flying and controlling it.
Cheers,
Pete J