Oh, that's a tough history! We had a cancer scare 4 years ago - there's nothing like seeing your family doctor in the emergency room and having him say "the prognosis is not good" to snap your priorities into focus. Best wishes on your plans.
There's a thread on here somewhere called "escaping the paradigm with $300,000" Sorry I can't find a link, I have pretty limited bandwidth here in Paradise. We bought an older boat for cash, smaller than what you're considering, ours is 33 ft, lived aboard while working and outfitting the boat with exactly what we wanted. Have been cruising since Oct 09 - no turning back. No interest in living on land again.