I tend to agree with you on Brady, as long as the Pats' internal knowledge of his health is consistent with warranting a long term deal; we just found out that Moss played with a separated shoulder (ouch!) this year and the Pats are far more secretive about Brady, so we really don't know.
I wouldn't be surprised if, as you suggest, Brady is looking to roll his current contract into a six year deal (up to the age of 39). I would be surprised if the Pats were to go out that far, but I can't imagine they wouldn't settle at four or five years with a lot of cash up front and a many incentives in later years. (IMHO, all bets are off if they don't get a deal done before next season begins.)
Hard to tell on Belichick. He's been an HC for 15 years. If you look at his peers (modern era HC's in the HOF), the 15 of them coached for an average of 17.5 years, so, if his contract goes through 2013, he would be right around that number at 19 years.
Shula and Landry coached the longest at 33 and 29 years, respectively. Lombardi, Madden, Walsh and Greasy Neal (whose career spanned the HOF's definition of the modern and pre-modern eras) retired after 10 years. BB's role model, Paul Brown, coached for 21 years (also spanning the modern and pre-modern eras).
So, it's anybody's guess. Since Belichick is so forthcoming about personal matters, I'm sure he'll be happy to answer any questions we might ask him.