@ASH556 covered a lot of what I could and almost all the points that I would have as well. This is my $0.02 from 10 years messing with these things for work and for personal use:
1) I've got 3 AR pistols. At first I thought they were gimmicks, but I have warmed up to them. To me, it's a viable means of vetting out a combination before I "render unto Caesar" for a Form 1 and/or travelling with a rifle caliber option.
2) If you can afford to feed a Blackout or use it for a specific purpose, then there's nothing wrong with it (I absolutely love the 110 & 120 Barnes loads). I tend to go towards a 9-9.5" .300 Blackout as personal preference. For 5.56, 11.5 or 12.5"
3) Sons of Liberty Gun Works, BCM, or FN would be my go-tos for a factory pistol. DD offers them as well (with factory Law folders as well), but I pass on them for personal reasons. Two of mine are built up franken-pistols, one is a franken-lower with an SR-15 CQB Upper. The latter will eventually get put on an SR-15 lower for an SBR. If you know what you're doing, build away. Otherwise I'd buy. My pistols pre-date the SBA3, so I'm adding those as I go along. The one thing I'd add to them all (and will eventually) is a Law Tactical side-folder.
It was previously addressed, but my information was 9" was the "sweet spot" for .300 Blackout.