I own a 2009 F350 6.4 Lariat w/Nav, I have about 97,500 miles on her. All of the emissions equipment is still in tact. Here is what I can tell you:
She is VERY thirsty. I have gotten as low as 7mpg towing an 8000lb trailer into a headwind. I probably average about 9 towing. On the highway empty I am ALL the way up to 13. Yes that is sarcasm. Around town empty I'm about 11. When I bought this truck I thought my mileage would be a lot higher.
Now there are internet reports of people who delete the emissions equip and add a tuner and they see their mileage increase. However, I also read that a majority of people who have catastrophic failures are the ones running deleted with a tuner. So I made the decision to keep it stock and eat the mileage for hopefully better reliability. We will see how that works out.
And while I haven't had any problems or issues with my truck (knocking on wood), I would recommend an extended warranty. Any type of repair is going to be "cab off" and expensive. Get the warranty.
There is a lot of good information out there. But my best advice is don't be afraid just because it is a 6.4 Ford. Navistar beefed up the 6.4 to correct the issues from the 6.0. There were radiator problems but I believe those were corrected by the 2009 model year. These trucks suffer from the 6.0 reputation and therefore don't have a huge demand. Which means that you have a lot of negotiation power.
The main problems with the 6.4 stem from emissions equipment. These are pre-DEF but post DPF. That is why they are so thirsty.
Change your filters early/often and you'll be fine. Just my opinion.
