It's not "impossible" to build your own gun once you're in the know.
Some of the small key parts are impossible to find so you'll need to be ready for that; others like the actual body are just hard to find.
Your best bet is to start out with a quality AEG that you can use and upgrade at a later date and then in tandem start building your own custom. You'll have the working model as a benchmark on hand.
Airsoft, where nothing is hurt but feelings.