a cheap lower will not stand up to as much abuse. other /arg/ types will disagree and tell you BCM, geissele, etc. are the only way to fly, and billet isn't as durable as forged, but you can get a nice forged lower for not much money these days. i don't see a lot of reason to get a lower more expensive than an aero m4e1 or some such, i'd feel pretty darn good about taking one of those into the field. i've heard some people say they've bought anderson and PSA lowers that were not well machined, so the hole sizings and positions are out of spec, etc. that would suck if you got one of those, but i consider the odds pretty low. if you're going to cheap out on one part, i'd say make it the lower receiver. just for chrissakes don't get a polymer, at least make sure its good t7075 aluminum.
upper receiver, we start to get into the precision machining being a little more important. the more consistent the lockup between upper and lower, upper and barrel, and upper and bcg, the better repeatable accuracy you're going to get and the smoother the bcg riding back and forth is going to be. that matters.
bcg matters a LOT. it's the "moving part" of the rifle and is subjected to the most mechanical stress. don't cheap out on your BCG. make sure it's cold hammer forged and magnetic particle inspected.
barrel also matters a lot. cold hammer forging and magnetic particle inspection are worthwhile there too, as is quality of the rifling and lining. this and the BCG are where I recommend getting a little extravagant.
triggers? i think there's considerable value to be had at mid point there, and diminishing returns with the really high end stuff. the geissele drop ins are not noticeably better than something like a velocity MPC, and frankly my favorite trigger is a JMT saber single stage, i've got mine tuned to probably 2.5 lbs and it's consistent as hell.
i have zero experience with zev uppers. i just can't say, although zev has a lot of experience making glock parts.