The DS285 servos are not good for a 40 size trainer. They are sub-micro servos with 17 ounces iof torque. Here is the description from Horizon "The new high-speed DS285 from JR® is perfect for micro heli users and precision electric foam airplane pilots."

If you get a DX7, get the one that comes with DS821 servos. They are 'standard size, high torque' digital servos with 72 ounces of torque. They are actually much more than you need but the DX7 packages only give two servo choices.