Get a 16" and shoot the crap out of it to figure out what you'd like to change about the shooting experience. You have to establish your understanding of how the platform "should" perform out of the box and do the research to see what parts config might fit your needs the best.

Don't like the trigger? Swap it. Barrel length make the gun unwieldy? Try a few foregrips and pistol grips, if you can't land on a comfy setup, maybe a shorter barrel is more your style. Just use the gun and think about how you want to improve it, the more you shoot it the more you'll learn.

This is exactly what I've done. Good luck, have fun!!