Dunno about you, but if I were the DA and the ME in his district, I'd be looking for a little antifreeze or rat poison in his system. That much money can make loving people do some pretty damn evil things.
I don't know about foul play but I bet a blood test for chemical stimulants would be interesting.
Not that I am implying anything...
sudden illnesses - There are several substances that will cause vital organs to "spontaneously" shut down several weeks after the substance has been ingested. At that time all traces of any foreign substances will be naturally eliminated from the system through digestive tracts, but the damage will be done, and the clock will be ticking. Your mark will be very unlikely to rush to the doctor for a minor abdominal pain. In fact, once he feels the pain, he no longer needs a doctor; he needs a mortician.
see: http://hitman.us/main.html