The trickiest part is building the upper receiver, which I have avoided like the plague. I don't like messing with the barrel extension, or even the gas system if I can avoid it. I waited for a special at Midway on an "economy" DPMS flat-top upper on sale for $300. You'd have to pay more just for the individual parts. I swapped out the charging handle, and installed one of those laser-engraved dust covers, both relatively nickel-dime parts.
Much more customization went into building up the lower. More custom parts can be used, and most are easy to get to. Also, not everything has to be done at one time.
The single most expensive assembly in the lower is the fire control mechanism. Even GI parts can vary in quality. The biggest danger is shallow hardening of the trigger parts, which can cause a nice trigger job to go bad. It's the choice of $150 for stock parts that might tune up nicely, or $250 for a ready-to-install target trigger.
Brownells has a complete set of videos on how to build an AR15 from the ground up. They also have a configurator to call out all the custom parts needed to build one that suits your purposes.
Bottom line: wait for a pre-built quality upper that does everything you want, and then carefully pick and choose the parts and gimmicks you want in your lower. That's how I built my last two "AR15s". One uses an AR57 upper and FN 50-round mags, the other is my M4E(economy) that I got with the sale price on the complete upper.
I did like the idea of the unitized trigger group you showed us during your m-forgery adventure.