Just for input, I think this needs to get moved to the optics section, but anyway. Dude, have you ever shot 1,000 yards? I ain't saying that a .223 can't go that far, but with the right rifle and the right load and people that have done long distance shooting for a long time, 800 yards is pushing it for accuracy. 800 yards is accomplished with hand loading bullet that are so long they will NOT fit in the magazine and must be loaded one at a time in most rifles chambered for .223. Are you going to be reloading? If you aren't, I don't think you can get it done with an 18" tube and factory ammo. Even with some hot handloads, I think you're gonna have heck getting that far in an 18" tube. Here's video of scotty taking his AR to 680 yards
http://www.youtube.com/watch?v=jJWLP81qCAE . This is one of him taking it 860 yards with some 75 grain bullets
http://www.youtube.com/watch?v=7B9NkQldeu0 .
Keep in mind that if you want to attempt something like this, you MUST have the proper twist rate to stabilize those longer bullets. Also realize that if you have that twist rate in your barrel, it will OVERSTABILIZE the lighter bullets. Yes, bullets can be overstabilized. What happens is that at the peak of the trajectory, the bullet will not nose over correctly and will fly nose up. The nose of the bullet will act like a wing in a way and cause a pressure differential that will cause a slight amount of lift. This means that after the peak of the trajectory, the bullet will hit high and right from what the calculations should show. It's not something that you want to happen or that will help with accuracy and some bullets might appear to keyhole on paper. You also have to realize that he's doing this with a 24" barrel. I'm not sure that you could get those heavy bullets that far with an 18" barrel. Just know what you're getting into. You can shoot that far with a semi auto rifle, just relialize that you're probably gonna ruin some brass and run into the complications of reloading for a semi auto. Alot of guys that use reloads in AR's also like to use adjustable gas blocks. I'm not an AR guy, so I don't know alot about them, just saying you might want to look into it and find out more about it.
You seem to have a good budget for a scope. Lots of guys will probably tell you to look into the Leupold Mark 4's, you can get a used one under $1K. You can get a new one if you look around enough in the $1100 range. Some people might mention looking for a used IOR for around $1K. You can find some lower down Swarovski's for under a grand and some Zeiss scopes too. Read about what you're getting into from people that have used them. I have known some guys with some Swarovski's that broke 10 shots into the scope. It can happen with any scope, but you just need to know what you're getting. Some people like them some people don't, from what I've read (don't have experience with them) some of them break. If you're looking at something in the mid range price range ($300-500), you could look at the Nikon Buckmasters, Bushnell Elite 4200's. I own a Millett LRS, it looks good and has some nice features, it has a 35mm tube and a 56mm front objective. Look in the optics for some pics and a little more info about it. I haven't shot with it yet because it's going on a rifle that I'm currently building, I'll let you know more after I got it on the rifle.
I don't want to burst your bubble about taking the .223 long range, just want you to know what you're getting into. Shooting to 1K has to be taken seriously, and has to be reloaded in the right rifle, otherwise there ain't a chance.
Edit: You might also look for some Schmidt & Benders. They're good glass, some of the cheaper ones won't have all the features of the more expensive ones or some of the same reticles, but they're considered a high end scope that the long distance guys like. I think that all the night force scopes are gonna be out of what you want to pay. I wouldn't waste my time with putting one on a .223 anyway.