MEK, I just built my AR this summer. I had not really seen or shot one for 20 or so years. After shooting one again, it became mandatory that I get one for myself.
Anywho, building the lower only presented one trouble situation. Getting the breakdown detent pins into place. They make a tool for it, which i did not have, that may make it easier. If you can follow simple directions and have any mechanical skill, building the lower is with in reason. MANY step by step instructions online. If you build your own lower, upgrading the trigger will not be anything you have not previously completed.
The upper on the other hand will need to be headspaced if you buy it piecemeal. There is a gunsmith in the family and this could have been an option but I chose to buy a complete upper with a lower kit. For simplicity, I'd advise buying a complete upper.
My build costs were $109 plus local tax for the lower, $420 for the upper and parts to complete my lower. Getting it all together, sans one retention pin, took me an hour or so.
My AR shoots as well as any I have used in the past, a Colt being my issued weapon for 3+ years.
If you ask me, the toughest part of building an AR is figuring out what to build, first....