My first AR was an experiment, built from the (then available) cheapest parts on Amazon; with one exception. I used a
Stag 16" (and NY Compliant) 3G match barrel. it was as cheap (except for the barrel), as I could make it. The whole point of the exercise was to see what was involved in the project and whether I could manage the trick.
Working off a Youtube video and the corresponding hard copy book, it went very quickly and easily, running about $350-$400 in/around 2005. I put a 6-24 Tasco Varmint/Target scope on it and it was surprisingly accurate for a first effort. I also used that project as my Wife-Excuse to obtain the more basic AR-related specialized tools.
Later I added a
Vortex SPARC AR 2 Red Dot and it became a very wieldy and surprisingly accurate gun for out to and beyond 100yd. It's very good for the task. I later found that 300yd and somewhat beyond is well within range, too.
Since then I've assembled another six AR's, including a pair in 6.5 Grendel and a .308. I learned a few things. Parts cost is not a primary concern, cheap pretty nearly always works good enough. The exceptions to this are a good barrel, a good trigger, as well as optics that can get you a clear sight picture. Expensive scopes are nice to have, but less expensive also works, just know their limitations.
By my own standards, there's a difference between the two terms "Build" and "Assemble". One assembles something from pre-made parts; but a builder makes those parts to begin with.
Optics are a tradeoff. First, there's cheap knockoffs; stay away from them. Next, there are pretty capable and readily affordable scopes. These are what I use. Then there are scopes which use features that exceed my needs and wallet. And finally, there are the ones that are very expensive. Some folks have the need and the skills for them, but many are simply bought for the bragging factor.
I don't fit into the last two categories, and I'm very likely the average shooter in those respects. The first category is what some refer to as 'The Poors', and some would include me, too. I think they'd be wrong, but then I'm not gunning for the gold medals, either. I shot in competition of about a decade, so I know my limitations.
I'm not a winner; but that doesn't trouble my sleep, either.
As a beginner, I think you'd do well to be in my category, at least at the start. There's is a certain minimum of skills needed in order to make effective use of the more pricey optics, and I seriously respect those who have them and can afford them. Getting the horse ahead of the cart in such circumstances makes it all too easy to damage really good equipment due to lack of experience. Save that for when you're in the top 10% of the pack, and can make best use of that gear.
Mainly understand that when the AR-15/M-16 was designed, Stoner configured it to be especially easy to work on, considering that most of the military who'd be wrenching on them would be HS Grads or less, your basic Draftee Types. Take confidence in that and relax into the process. Look up the basic literature, get in some basic learning, buy the tools they recommend/use for the basic assembly process, and be at it. Having a mentor to help never hurts, either.
Remember, keeping it as simple as possible is a major asset.
Greg