Most 5.56 ARs will have aluminum receivers. Even the cheap ones from good budget manufacturers like Ruger, S&W, etc. Avoid Del-ton, Diamondback, DPMS, anything owned or recently owned by Freedom Group. From anecdotes, QC isn't their strong suit.
That Sig is a nice package if you just want everything in one go. Fancy lower receivers aren't a necessity, especially if you're right handed and aren't doing tactical moves like firing from your support side. But having those ambidextrous controls and built-in QD sling mounts are nice touches. What is really nice is having an upgraded trigger. I don't know how that specific trigger is, but such upgrades usually run about $100-150 at a minimum.
The Sig Romeo5 it comes with is a pretty good red dot, especially for its price range. It's fairly durable, lightweight, has a very long battery life (4 years+ on) which is extended with the motion sensing "shake awake" feature, and holds zero well. It's at the bottom of optics I would use on an AR aside from irons, but they're a solid choice especially if your budget runs out in the low 4-digits. Of course they can bought separately and mounted to any AR you get, doesn't have to come with the rifle. They can be had for about $120-130. You can always upgrade later if you want. Something to consider if your budget won't fit something $400+ which is what I consider the next tier of good red dots (Aimpoint PRO, Trijicon MRO, EOTech 512).
Aside from maybe getting lucky at a local gun shop, Sportsmans is a pretty good place online to shop if you're within acceptable distance of their store in Coon Rapids. Search their Modern Sporting Rifles category if you can stand getting a 16" barrel, or if you want a pistol, hit their Handguns category and filter by "Modern Sporting Pistols" and then hit the exclude out of stock filter.