Upper is the complete top half of the gun, ie, barrel, barrel extension, upper receiver, delta ring assembly (if standard config), front sight/gas block, forward assist, dust cover. A comptete upper will have the bolt group, other wise it's commonly called an "upper half".
Lower is the lower receiver, fire control group,
receiver extension, buttstock, buffer assembly, etc.
A stripped lower is the actual receiver only, it's just a hunk of aluminum that's barely recognizable as part of an AR, in that it has no stock, pistol grip, etc. (usually). You'll have to install the lower parts (bolt release, takedown pins, buffer tube, spring, etc). When you hear that people "built" their lower, it usually means they assembled the lower receiver parts kit into the lower receiver, ending up with a complete lower.
You can buy a complete lower from one manufacturer, and an upper from another, and just mate them up, push the takedown pins back in, and go shooting. (Colt lower/uppers have a different pin size, so they're not compatible with everything...I don'k own any Colt stuff, so I don't know exactly what's what.
There are several ways to get your 1st AR...buy one complete; buy upper and lower separately, buy a kit (which has everything BUT the lower receiver), or just buy the individual components and build it yourself.
The lower receiver is the "firearm" here...so you can have everything else, including kits and complete uppers, sent right to your front door.
It makes is very easy to get what you want, but also can overwhelm someone new due to all the choices.
Assembling a lower is easier than you might think, there are absolutely fool proof directions on this sight. One or 2 specialty tools will make the job eaiser, but they're not required. I've built a couple, and I'm far from the most mechanically inclinced person in the world.
I't recommend a 16" flat top AR, with no muzzle brake. A muzzle brake will complicate things down the road should you want to install a float tube, remove the front sight, etc. Of course, if you get the barel config you want from the get go, you probably won't have to change anything, but still...down the road, I almost guarantee you'll have more than one rifle, or upper at least, and you'll be thinking about swapping parts...trust me hehehe.
The Bushy Modular rifle is a nice looking gun...but you could build something *similar* for about $200 less if you were so inclined, even more if you were willing to give up the fold down front sight.
There's a lot to be said for building a rifle in terms of a learning experience, and you can indeed save some money if you're on a tight budget. Either way, you'll love your AR, and will have a buttload of fun shooting it, and figuring out ways to pimp it out...whether or not you ever follow through on them lol.
Just getting the upper and lower separately can save you money (no excise tax), and once again, you can be a little more choosy as to what you get.
My first rifle was an Armalite 20" HBAR A4 upper, and an Eagle complete lower with the National Match trigger. I'm VERY happy I decided to go with the NM trigger; the rifle worked out to be the same price as a complete Armalite rifle minus the match trigger. Thousands of rounds later, the NM trigger is still going strong.
Well, as you can see, there are lots of choices. The first one is budget, then barrel length, and/or upper config. But, with all the cool stocks, that's no easy decision either lol. Good Luck!