RE: Questions regarding Futaba Servos
The S3305 is recommended for the rudder of the UCanDo-60. Metal gears and 99 ounces of torque at 4.8 volts. GP recommends 'standard ball bearing' servos everywhere else and the S3004 is a standard BB servo.
You could use the S3001 in place of the S3004s for a fairly small price difference. The S3001 is called a 'precision servo' and I've seen a description of the differences between it and the S3004 but cannot remember much about it. The speed and torque are very similar.
Stronger and/or faster servos will not do anything bad to the plane but they hurt the wallet.
You can always go coreless $$$$, or digital $$$$, or coreless digital $$$$$$$$$$
I guess the only way to know if you "need" coreless digitals would be to install regular servos that meet the airframe's requirements and fly. Then install the coreless digitals that meet the requirements and fly. If you cannot tell any difference you do not need the high dollar servos. If you can tell a difference, and you like the way the high dollar stuff works, you have to decide if the price difference is worth it to you.