I’ve built a few ARs - started with plain jane, then one with a few features, then fully tricked out. My suggestion: get what you want on the first one, or just keep upgrading. It can be an expensive hobby… cheaper to buy Legos 
That said, I’m considering an AR-10 build. It doesn’t look as straightforward as the AR-15. Maybe I’ll just try and trade up one of my AR-15s for a factory built AR-10?