Welcome to ARFcom pavo.
I ain't much of an expert when it comes to building ARs, I've only put together a couple lowers. But if you're looking for an accurate AR, and your new to the little black rifles...It might be best to just buy a Varminter or a V-match. (Or some other brand's accurate offering)
I'd say if some of the pros here don't chime in, your best bet is to buy a Varminter/V-Match or other brands' accuracy offering. The lower is pretty simple to build but the upper is obviously where some added tool expenses, caution, and tighter tolerances must be observed. I know if I had built my first AR from scratch, I would have messed it up since I'm impatient.
I own a BM Varminter and can't say that I see much of a difference between it and my old V-Match. Both are accurate. The Varminter just has a slight edge with the cool factor and came standard with the two stage competition trigger installed. (The Varminter looks especially freaky with an ACE skeleton stock on the rear...) If I were you, and unless I have someone to coach me through the build, I would just purchase my first for several reasons, one being the warranty since even the big manufacturers make mistakes here and there. (That way after you get it, tear it down and learn it good at your own pace)
If you want to have a little pride in having built your own AR, I guess it still counts to build just your lower and slap a complete upper on it.(I've been there before) There's still that little bit of pride in knowing that you brought the receiver to life and there's nothing wrong with that. Also you can install whatever trigger you want, grip, springs, etc. etc. These little things add up pretty fast. I have several sets of Mil-spec Fire Control Parts laying around in my constantly expanding replacement tub that have been replaced by up-grades. I also seem to be accumulating grips that I replaced with something a little better, selectors that weren't ambidextrous, mag-catch that wasn't ambi. etc, etc, etc, etc. In fact, it was this large collection of spare parts, which were in duplicate, that lead me to build one of my first lowers. The parts were there, so why not?
BTW, the Varminter has a 24" barrel. Not sure if you were dead set on the 20" but if so, the V-Match has a 20" and if you like the barrel fluting, you can get it done on the V-Match as well for a nominal fee. The V-Match has a chrome lined barrel where-as the Varminter is unlined. Either way you can add a carry handle, but you would need to either put a flip up front site, a fixed front site or a picatinny gas block. The flip is expensive, the fixed site/block can be a pain. Either way good luck.
Take care.
Sly