A DPMS Panther or Kraken could be used as a "3D repair guide" to get your hands dirty on.
Also having general knowledge of mechanical systems and how the parts interact with each other is a good start too, and it wouldn't hurt to read up on materials properties if you want to get into more advanced stuff.
tl;dr AEG - electrical system which uses electrical potential energy to spin a motor to spin gears to pull a spring and piston system back and releases it to compress some air to propel a plastic projectile. Everything else is just extra parts.
ಠ_ಠLess QQ more Pew Pew
READY TO >> RACE