For the most part, a lower is a lower is a lower. The stripped uppers and lowers are all basically identical. As long as you get an aluminium one, I wouldn't worry much and would buy the cheapest. For parts that go in a lower, it's up to you. I don't think you would notice a bit of difference from one A2 style fixed stock to another, and if you are looking at the standard 6 position collapsible stock, again I doubt you notice a difference from one to another. The little parts, like the pins and springs are all generic. Then there is the trigger. Most standard GI style single stage trigger is average at best, gritty and a bit heavy with some creep. It's hard to get much more than the run of the mill standard trigger on a $700 budget. If you get a collapsible stock, it is proper to stake the castle nut (nut that holds the stock tight). This keeps everything from backing out.
The upper half is a bit more complex. There are a few things that make a rifle run reliable and there are a few others that make the rifle accurate. On a budget you will have to skip some of these items, but you can typically still end up with a reliable and fairly accurate rifle.
A top end upper half will have a proper buffer and a quality buffer spring for the gas system/length on their upper. It will have a bolt that is pressure tested. The BCG will also be properly staked (screws that hold the gas key on will be punched so they can't back out) and the bolt will have a quality extractor, spring, and potentially oring. The feed ramps on the barrel extension will be the same style as the receiver. The barrel will be a top quality barrel, will have a properly sized gas hole, and will have a properly mounted FSB. If it is a "defense" based rifle it is going to be chrome lined and of milspec grade steel. From there, things like sights and handguards are all personal preference.
The budget type rifles skip a few of these points. First, they typically don't test their bolts. Testing is more of a QC insurance. If you need every round to go bang and can't live with the chance that a bolt fails early, buying a tested bolt is beneficial. For most of us, it isn't a big deal either way. You also see that many of the package deals don't pay much attention to which buffer they toss in compared to the gas system it's being used with. You see staking on the bolt being superficial and improper, though that is easily fixable with a hammer and a punch. The extractor/spring/oring on these budget rifles typically don't measure up to those on a top tier rifle, but again, it's a cheap and easy fix if anything goes wrong down the line. The budget rifles use pretty standard basic handguards, which as the same as those used on the base models from top tier manufacturers. The barrel is another issue. Most of the time, the feed ramps on the barrel extension match the receiver any more. You do see steel of "lesser" quality being used on the cheaper rifles as well as a lack of chrome lining. The difference in steel type for the average user is insignificant. For those that shoot until a rifle is glowing, it makes more difference. For the guy shooting a bit on the weekends, not so much. You do see some budget rifles drilling the wrong size gas hole in a barrel for the system they use which can make it a little more difficult to get the rifle extremely reliable.
In the end, to know what features some budget brands have and don't have, you have to research the brands that interest you. Some are better than others. Some cut fewer corners than others. It's all about looking at what is in front of you, knowing what you demand to be done on a rifle, and what you can live without, and going from there.
I think there is a lot to be said for the S&W sport. It's a rifle that has a properly built BCG that is also tested. It also has a barrel that is treated to be all but as durable as a chrome lined barrel without the issue of losing accuracy due to the chrome. It's assembled by S&W, who seem to build a good rifle and it has their warranty, which is very good, in the case that something goes wrong.
The kits may save you a little money over the S&W sport, if you build a similar rifle. The advantage of the build it yourself rifle is that you can go in, buy the stock you want, the trigger you want, the handguard you want, the barrel you want, and the BCG you want. There is no compromise to the parts you select. You end up with a more personalized rifle than the standard carbine. Say you want a midlength setup with a given float tube and butt stock. You go to del-ton, pick the options you want, and then it comes to you with them. No buying a factory rifle to then buy extra parts. That's where the benefit of a build comes. If you want an off the shelf ar, buying one from a factory is pretty similar to building.
