I'm certainly not a gunsmith but I'm comfortable taking apart my AR - still looking forward to being corrected on any number of things below by the real experts here.
Schematics are readily available, but they can almost make it look more complex than it is. In my limited experience, I don't consider the AR platform tool-heavy, with punches being the most common tool(s) necessary. If you want to "see," here's a pretty thorough thread on builds:
http://www.ar15.com/forums/topic.html?b=3&f=4&t=226782
This likely influences its popularity - like the 1911, parts manufacturers know it's a solid, tinker-friendly, relatively inexpensive (compared to other Western service rifles) system.
You push in two takedown pins to separate the receivers (upper/lower). The lower is pretty simple - the trigger group doesn't need regular disassembly, but there's plenty of info out there that will be enough for most laypersons to do so if they please. You can also buy pre-built, drop-in trigger groups if you'd like something nice/aftermarket without the trouble. External controls aren't complicated or expensive to replace with oversize/ambidextrous/etc. parts. Removing the buffer tube and spring is just pushing down a spring-loaded retainer. Stocks require a wrench for removal, but again, it's nothing sophisticated. There's boatloads of these, cheap and expensive, the "coolest" ones in my opinion made by Magpul and CAA. Not much to the pistol grip either, and also lots of options.
Regarding the upper, a flat-top gives you a lot of options for accessorizing, although the marketplace is full of mounts and add-ons for a standard A2 upper as well. A2 irons and aftermarket flat-top sights aren't difficult to play with as long as you're not prone to losing small parts, or keep spares on hand knowing you're as clumsy as me and will drop them somewhere you can't reach. The the bolt carrier group and charging handle come right out once you take the rifle down. The bolt carrier and bolt are easy to disassemble and reassemble, although you would probably buy a whole new group if you were upgrading. Depressing the delta ring will get the handguards off, which you can replace with railed guards for accessories. Changing barrels gets into a little bit more effort, but it doesn't necessitate a gunsmith. You'd also need to do that if you want to free-float a rail or handguard system - however, for 200yd accuracy, that's probably not a concern. The gas system is pretty much hands-off - doesn't require much regular maintenance, and as long as your's is working alright from the start, you'd probably just keep what you have until you replace the upper.
Which is also what's nice - if you decide you want a tack-driver later on, but don't want to buy a whole new rifle, you can just pick up a new upper (or lower for use with a versatile upper). A well-built lower - or a not-so-well-built one if you're just concerned with it going bang at short range - accepts almost whatever upper you slap on it, including a number of uppers that have been designed for other calibers. There are also conversion kits for the existing upper. A lot of gun stores have parts on hand if you want to change out something, and many online gun sellers have a dozen makes, finishes and variants of every part available.
What'd I miss out of your topics - mags are easy and cheap, and haven't been finicky in my experience. I've gotten a handful of cheap magazines ($10 a pop) from various online sellers - used polymer, steel, etc. - no problems to speak of. Standard AR-15 function doesn't use an op rod, it's all gas. Pre-and-post ban uppers usually come with a bayonet lug, and you can get quad-mounts for the gas block if you want to forward-accessorize without full railed handguards. Flash hider is just screwed off, although some target barrels are not threaded. No receiver covers, but you do have the ejection port "door."
Anyway, hope that's at least a little helpful. And I don't mean it if I sound pushy - I'm not saying the Kel-Tec or other rifles wouldn't fit your bill - I just didn't think the AR should be "crossed off" the list if you're more into semi-autos than bolt guns in .223 Rem.
