For the least "swimming upstream" going with a M4-like AR in 5.56 (16" barrel, adjustable stock, flat-top upper) will likely be the ticket. Would look a bit like this:
That would let you narrow down the choices a bit to fore-end versus rails and price versus availability.
Get used to that rig, then decide if you want to go with free-float, "locking" formats, and the like.
Start with the less-expensive 20 round mags, too; you can later let price/availability determine if you need/want 30s.
Also, please note that price is not always the best indicator of what you are getting New-in-the-box. There's a substantial price point difference between, say, the S&W M&P and a Colt LE6920--ask which one is "better" and you'll get a flame war on near every forum you ask upon.
Get used to what's in the box, first. Later you can decide if a c-note's worth of trigger is worth it
to you (ok, probably is, but, that's just my 2¢). Decide you really need an M-loc freefloat fore end instead of carbine clamshells? That's maybe an hour's labor to swap and whatever the tube costs you. maybe you want a cheesegrater fore-end--about the same.
Now, maybe, you'd like even faster 'gratification'--you can buy a whole upper in all sorts of configurations of forer ends, muzzle devices, the works. That will be around 2 to 3x the price, but it's the 60-70 seconds' it takes to pull the pins and make the swap.
Now, if you get bitten by "the bug"--all bets are off. You will wind up with random magazine pins, bolt release gizmos, charging handles, and the like. It's Ok, we all have those [
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