An RRA NM model is probably the best deal going for a complete, box stock service rifle AR15 from a big manufacturer.
For something higher up the food chain, Compass Lake and White Oak seem to dominate the line. John Holliger or Frank White will take good care of you. Shouldering a CLE space gun with a White Oak buttstock, I can certainly see the appeal of 'em. The White Oak stock is the cat's ass. John or Frank can rig a space gun up to your heart's desires, right handed bolt release, slot the receiver/carrier for a charging handle, make 'em in all kinds of pretty colors, the sky's (and your credit card's) the limit.
Sights? I'm not a match rifle guy, so I don't know the first thing about 'em except they got too many knobs and adapters and thingies that screw into 'em for a dumb ol' service rifle shooter like me.
The one everyone talks about is the Warner. Others include RPA, Centra, PNW, Phoenix Precision, and I'm sure I'm forgetting one or two. I see that David Tubb is building his own rear sight now too. Front sights, 22mm ladder sights - like the RPA - seem to be the most common, with 30mm sights here and there like the Stallings Machine Rightsight and Ross Beer Can.
Calibers? For AR15 space guns, there's something called a 6mmAR that seems to be getting a lot of praise (I think the website is 6mmar.com). There's only so much you can fit in an AR15 platform. .223 Ackley Improved is an option. I've read of more radical things, like .22 PPC, but they take a lot of work to get right. Of course if you're shooting a service rifle AR15, it's .223, all day, every day. If you're wanting to shoot a bolt rifle, there's all kinds of fun high horsepower cartridges you can use to sling bullets downrange. Tubb guns are a neat alternative, but like Jon said, you'll be out $5K or so to get a Tubb gun and everything you need to make it work. Another option is a Tube gun, which is basically a sleeved, magazine fed receiver that you glue a short action Remington 700 in to. No bedding to worry about, bolt runs in the stock like a Tubb gun so I think you can keep your face in place as you run the bolt during rapids. MAK Enterprises and Gary Eliseo both make tube gun kits; the latter though is biased towards the 6BR cartridge due to it's magazine design to enable feeding those fat stubby little cases. And of course, there's always conventional stocked bolt rifles like Remington 700s, Winchester 70s and (if you can still find them; check with Mac Tilton) Tikka 595s.
There's enough stuff out there to give you a headache if you sit back and look at it for a while.