ARs are not difficult guns to build (and technically speaking “assemble” is probably a better word to use than “build”). If you don’t have much prior experience with handling an AR, it could be a little tougher just due to not having time with a fully complete and working model, but with enough research you can alleviate that concern.
I recommend answering a few questions (at least for yourself) prior to building. There’s a lot of ground that can be covered between different variants of ARs. Are you looking for just a middle of the road “normal” AR, or do you want something along the lines of a class A1 or A2, or something a bit more specialized for closer range or longer range? Are you ok with a 4 MOA rifle, or a 2 MOA rifle, or do are you not happy unless you can get sub-MOA?
I know you said essentially “less expensive…just works” but “just works” for a box of ammo at the range once a year and “just works” for several weekend carbine classes where you’ll be shooting 1500 rounds in 2 days in each class are different in costs, components, and even some build techniques.
Anyway, general recommendations:
The lower receiver is defined as the “firearm” for the AR design and is the only part that needs to go through a FFL. If you’re building, you’ll want to buy a stripped lower. Forged is the normal for AR receivers, but there are billet receivers available (usually a matched set of upper and lower). Nothing wrong with forged receivers - the only time I’d go with a billet set is if I wanted to wring out every last bit of accuracy (which is not to say that a good forged upper isn’t able to be accurate), wanted a monolithic upper/rail, or just wanted to have one for the hell of it.
Get the right tools. It is certainly possible to build an AR without them (especially the lower) but the right tools make the process so much more enjoyable. A good set of roll pin punches, a set of roll pin starter punches, a tool for installing the front pivot pin detent, and a wrench for the castle nut are all good ideas.
For the upper there are different tools you’ll want. The main ones are a way to hold an upper receiver in a vise for barrel install, and a wrench for the barrel nut (which can be different depending on which rail you go with).
A torque wrench is recommended for the barrel nut, but the torque spec is pretty wide so you can get away with not having one. Depending on the barrel/gas block setup, you may want a set of feeler gauges for the right offset off the shoulder behind the gas block journal. And again, depending on barrel/gas block/planned use, you may want to dimple the barrel for the gas block if it’s not already done, or even cross pin the gas block. You can get different jigs for those, and I’d recommend using a mill but a drill press can usually suffice.
On the barrel, the headspace is set by the barrel extension. The vast majority of the time Company A’s barrel will headspace correctly with Company B’s bolt, but it’s good to check.
Lots of people think of the AR as a piston-less design, but there is effectively a piston internal to the bolt. That means the dimensions of the bolt compared to the bolt carrier are pretty important to the function of the rifle. Don’t skimp on the bolt carrier group.
Overall it’s not really difficult to build an ok rifle, but if you want a really good rifle there are a lot of little things that can go into it.
I’d recommend looking through School of the American Rifle videos on YouTube - he has a lot of really good, detailed information on the design and function of the AR.