Usually, AEGs dont require much maintenance other than cleaning/oiling the barrel and maybe oiling the magazine spring once in a while. That's why they're great starter guns. Just keep the battery charged and you're good to go.
Depending on the gun, changing out inner barrels is usually not too difficult. Getting to the gearbox is a little more complicated, but there are tutorials on here/youtube/other sites that should be able to walk you through it.
If you want a gun that you can strip down and clean, get a gas powered pistol/rifle.
