With respect to either buying versus building a custom AR: there are only a few high end AR's I would buy. Spikes Tactical, Daniel's Defense, LWRC, Black Rain. LWRC being the absolute king of the AR's out of the box. never get a Direct gas impingement when you can get a piston drive system. LWRC and Adam's Arms both offer Piston systems. I have worked with both. I build Custom AR's in my little gunsmith shop. I always ask if they want reliability or lesser expensive. you would be surprised at the looks I get. if they want reliability then I always use Adam's arms or Superlative Arms piston drive kits on my custom AR's. only when they want to save money do I use direct gas impingement.
ask yourself this question: do I want the fun of picking out all my parts, making it almost the exact way I want it and am I willing to pay upwards to $4,500.00 USD for an AR that has everything and all the right parts on it? If not, then buy a reputable brand name and up-grade the base model when you find out which parts you really want/need on the gun. by that time you will have worked with it a bit and will know what parts, what extended levers you need, and how you and your gun need to interface.
Yes that is part of my job. making customer understand what they want and how to get it. The best way to find out what you like and what accessories, levers, and such you want on your AR.. fondle as many AR's as possible with all the cool stuff on them.. make note of the things that make each of the AR's good and bad, then start a list. it will help you pick out the right base model and what to put on or delete from yours.