Hey @Brad the amount of options you have can be a bit overwhelming at first, but dont get discouraged. The AR15 platform is like a bunch of legos. I’ll give it a shot and see if I can simplify things for you
The first question is, pistol or rifle
TLDR -> Get an AR15 with a 16" barrel
More info -> The full answer is “it depends” . Your first AR, you should probably get a rifle with a 16" barrel because it’s just “the easiest”.
AR Pistols are convenient because of their shorter overall length and accompanying lighter weight, making them handier for home defense. Easier to travel with (IANAL, state laws vary, but you can take pistols sometimes where you cant a rifle), fit in smaller bags so your nosy neighbors dont know its a rifle in there, sometimes cheaper cost (less metal used), etc.
On the other hand, laws can sometimes be funky regarding AR Pistols (thanks ATF). Pistol brace rules can change, you cant use foregrips, sometimes concerns with the length of certain braces. 5.56/.223 ammo is often velocity dependent, so something that works really well out of a 16" barrel doesn’t work so well out of an 11.5" barrel.
There are also edge cases like a 14.5" or 13.7" barrel with a pinned and welded muzzle device. Skip these too for now
Its not impossible to navigate, but for someone’s first AR, having just a regular rifle is less to learn about and you can just use it and have fun out of the gate.
I would like the option for changing out parts in the future, particularly for 9mm or even .22lr use.
TLDR -> I wouldnt put 9mm/.22LR on your list for your first AR15
More info -> 9mm and .22LR require more than just a barrel or upper swap. They need specific bolt/bolt carriers, and usually a different lower as well. Some calibers like 300Blackout or 6.5 you can just swap an upper (which includes the Bolt/Carrier and barrel), and others like .308, 9mm, .22LR that require a different lower as well as an upper. Their similarity is that all the controls are in the same place and they work the same way as the AR15. Here’s a good list of alternative calibers. S&W has a .22LR version that looks and works exactly like its 5.56/.223 counterpart with the advantage of significantly lower shooting costs, but AFAIK the important parts are not interchangeable.
Now, all the other stuff you can swap out to your hearts content (usually, you’ve already found out about Ruger’s AR556). It really is like a bunch of legos.
The second question is about which make/model
TLDR -> In order of cost, Palmetto State, S&W M&P Sport, Colt, Bravo Company. Higher end -> Daniel Defense, KAC, POF, Sons of Liberty GW
More info -> There are more on this list, but you cant go wrong with this shorter list.
You want to avoid “cheap”, but inexpensive is OK. If you literally want the least expensive, but still sturdy enough to rely on I’d start and stop on Palmetto State Armory (PSA). You can buy a complete rifle for < $500. You can save a little bit of money if you buy the lower and upper separately, you literally just put the two halves together yourself by pressing together and pushing in two pins (literally thats it). You can save a little more money than that by getting the upper, stripped lower, and a lower parts kit but now you are building a lot more yourself.
In the sub-$1000 category there is the tried and true S&W M&P Sport series. If you can find one without being price-gouged the Colt 69XX series is fantastic (Colt has stopped civilian production, so the supply is drying up driving up cost). You can also look at Sig’s TREAD line and the Ruger AR556 but be wary of proprietary parts if you want to change things later. They are reliable though and very reasonably priced.
Bravo Company (commonly referred to as BCM) is among the gold standards in AR platform, IMO. Cost will be in the ballpark of $1000+, but along with Colt (lower) and Daniel Defense (higher) is a known and proven provider to law enforcement and militaries. You can trust your life to these for sure.
What do you get for paying more money? In most cases you get materials or workmanship that will last longer overall or last longer under higher-stress environments like full-auto (which you can’t do anyway). For a low-volume beginning shooter, does it matter that the one you buy will only last 10k rounds instead of 20k rounds before you need to replace a barrel or bolt? Probably not. In some other cases you might get better accuracy, but if you aren’t pulling competition or shooting out past a few hundred yards or trying to put really tiny groups on paper then this might not matter either. In some cases (especially the higher end brands) you are paying for more exotic materials like titanium that give some benefit like lighter weight for same/more strength or coatings on some parts like Nickel Boron on the bolt carrier, etc. And sometimes you are just paying for the brand name.
You have to make the dollar choice that makes sense to you. We aren’t fighting in Afghanistan, but there’s nothing wrong with getting a rifle that can survive that if its in your budget. But there’s a lot to be said for getting an inexpensive rifle and spending the savings on training and ammo.
YMMV, hope that helps!