Hey there. I'm new to the AR world but I'm waiting to get my 20'' A2 rifle kit soon and plan to make it a good long range gun. I've been researching trying to find what scope I should get but don't know how far I should expect my rifle to reach. I've heard around 900 yards but don't know how true that is. Anyone got some good advice to throw my way?

A2?  With a carry handle?  You're wasting your time trying to put long range optics on that upper.  Would be perfect for Service Rifle assuming that it has a good barrel and a hidden float tube for the hand guards.  All you would need there is a good rear sight.

Also, what do you expect it to be good for at long range? Varmints/paper/other (your answer shouldn't include deer)? The M-16 (which has a 20" barrel) is rated by the military as having an effective range of 550 meters. Accuracy is only part of that. The bullet still needs to be able to cause damage once it gets there in order to be considered effective.
If you're just looking to shoot paper, the platform is capable out to 1000 yards with the right parts (barrel being the first thing that comes to mind). Depending on how the rifle is built, you may get anything from .5 MOA (or mabye a bit less) to 2 or 3 MOA accuracy.
I agree with the guy above regarding the A2 configuration. If you're mounting glass, it should be on a flattop (M4 style upper receiver). As far as scopes go, I'm partial to Nightforce. Recommend clarifying your intended use.

I don't have a specific use I'm going for. Kind of an all around gun. Primarily paper and to have an ''in case SHTF'' gun around. But i was also thinking about maybe hunting next year. .223 is an alright hunting round is it not? And not that i'm arguing, just wondering, could you specify why it is impractical to mount a scope on top of a carry handle?

From the Lyman reloading manual:
"In addition to its military service, the cartridge has been used for varmints, small game and target shooting with great success along with use by law enforcement agencies. The 223 Remington should not be considered a deer cartridge. Many states mandate the 6mm/.243" bore size as the minimum caliber for whitetails."

Hornady manual:
"The 223 Remington is an excellent choice for varmint hunters."

Nosler manual:
"Some will look down their nose at those who use 22 calibers for hunting deer. Most assuredly, this disdain is due in part to lack of experience with Nosler's smallest Partition. I have conducted extensive bullet expansion and penetration test with this bullet and at 100-yard impact velocities it will perform on par with any 30-30 Winchester load even if bone is struck." [...] "However, the true forte of the 223 Remington is predators and varmints..."

Speer:
Doesn't mention deer. Warns against the use of lightweight varmint bullets in barrels (like many ARs) that have 1-in-7" twist, because the rotational speed can rip them apart.

So I would say if you are an experienced hunter, or are going to be hunting smallish deer at closer ranges, you'd probably be fine. But the ability of the bullet to kill game is going to go down with range, as the bullet loses energy. You don't want to wound an animal that is 600 yards away because you didn't use enough gun.

The flat top is easier, stable, consistent and offers a lower height to mount the scope. Long range is challenging enough without adding complications that could be avoided by going with a flat top. You can also put a removable carrying handle on it if you want. The picatinny rail is designed for optics; on the carry handle they are an afterthought. It's not necessarily impractical, just less practical.

Thank you. I love it when I actually get good info. Some people i've talked to in my town just kinda say ''sure I dunno why you couldn't hunt with a .223''. With the knowledge that i'll probably not be hunting with my rifle, could anyone suggest any good scopes for maybe closer to 3-500 yards? I'm not the most knowledgeable guy with ARs but can get by on what I know, but I'm even less experienced in optics. Any exact models I should look into? (Also I'm running off of a Min. Wage budget so I'm thinking in the 3 to 400 dollar range if thats do-able)

Look at the nikon prostaff series, or the redfields. Both are good quality for the money, and carry good warranties.  Both have scopes to fit most applications. Magnification is going to depend on what you specifically want to accomplish. A 3-9x is a safe bet for most hunting, but if it is going to be long distances on small game, you might want a 4-12x.

Nikon 3-9x40 on sale @ Cabela's for $99.. down from $159..

Just ordered one for my DPMS 308 for the same reasoning... 300-450 yards.

You are going to find any attempt to use a scope on a carry handle upper awkward at best.  The scope is going to be WAY too high for a proper cheek weld.  That means inconsistency and essentially rules out any degree of usefulness at any king of range.  If you're committed to keeping and using this particular upper, get good irons and get good at shooting with them.

Thanks for the input. As much as I love the A2 I think I might just go with the flat top 20'' and go modern with it, then just make my next build an A2 Dissipator to cut out this problem of the scope on top of the carry handle. Are flip up iron sights for back-up as reliable as standard iron sights?