i was under the impression that an AR needed parts replaced after just 1000k rounds? and also was told that after 100 rounds its so dirty you start getting FTF's?
Whoever gave you that impression/told you that doesn't know much about AR15s and probably should be avoided as a source of information.
You keep mentioning "1000k rounds". Typically "k" is an abbreviation for "1,000". 1,000k would be 1,000,000 rounds - which is probably significantly beyond the design limits of any small arm manufactured today even under ideal conditions.
Like any machine, parts replacement depends on use. It is very difficult to pin down reliable parts replacement numbers for AR15s because:
A) Not everybody uses the same quality of parts in their AR builds and some parts are better than others
B) Everybody has different shooting schedules - 10,000 rounds fired over 10 years with the barrel never heating past 150F is going to show a lot different wear than 10,000 rounds fired in a single day - which will likely destroy at least some of the rifle parts.
Having said that, with good quality parts, you can usually replace the bolt when you replace the barrel.
As far as after 100 rounds it is do dirty it stops functioning properly, that is just BS. If a rifle is doing that, it is a bad/broken rifle. A properly built AR will not do that. In classes I have run mine 450-750 rounds in a day with no cleaning or additional lubrication added.
As far as cleaning, I believe Pat Rogers has several direct impingement ARs that are now past 10,000 rounds with no cleaning of any kind. They just periodically add more lube. One of them is even at 28,000+ rounds with no cleaning (though it has had parts replacement). So an AR will definitely run dirty - they will get pretty filthy run like that; but they will run as long as they are well-lubed.
As far as the rifles you are looking at:
The SIG 556 has an AK-style gas piston system. Of the three you mentioned, it probably has the most proven track record.
The Ruger SR556 is a new entry and not a lot is known about it other than it seems to be having the teething issues typical of a new rifle. Trying to wedge a gas piston system into a space designed for a direct impingement system is actually a good engineering challenge and presents new problems.
I have no experience with the SU-16C, so I won't comment. What little experience I do have with Keltec tells me that they are not in the same class as the other two rifles.