Try downloading via Software Update again, this time use the "Show Details" and uncheck, download and install in repeated segments, leaving the largest download for last.
Spread out the downloads over time and during off hours.
Also you have the newest machine, sometimes Apple discovers errors it has made, thus sends out a command to Akamai servers (I'm guessing here) to stop downloads for particular machines, however it's fine to download for other machines.
If after downloading all the other updates you can't download one particular update, give it some time and try again a few days later.
You should run a hardware check on that machine, and if you got a hard drive, perhaps you should consider reinstalling the OS due to a bad factor install, although if you have a slow network this also could be a pain.