It is likely cheaper in the long run to buy the 6.7. The 6.4 was a mistake and ford knows it. That's why that engine didn't stick around for more than 2-3 years.
If you are not going to be towing then why would you want the added maintainince, fuel, and initial costs of a diesel? If your primary concern is reliability in a diesel then you should really look at the cummins. Or just get a decked out 1/2 ton and spend the leftover cash on wheels/tires etc.
I've driven or owned all of the big three's diesel options. I've had a 5.9 12v, 5.9 24v, 6.5 Detroit, 6.2 Detroit, LBZ duramax, LMM Duramax, and now I have my 6.0 Powerstroke, my brother had a 7.3 powerstroke. I've either owned or spent plenty of time in darn near every brand of diesel light duty truck. The cummins is not the epitomy of reliability unless you plan on getting an early 90's mechanical. The newer cummins are plagued with the same problems all the other trucks are because of the emmisions.
If you want the end all be all of reliability you want a 96-98 f250 or f350 with the 7.3 in it. In my opinion they are the longest lasting engines I've ever seen.
I would agree there were major problems with the 6.4 but thats not why it got cut so short in life. Ford had already made plans to drop out of their deal with Navistar and start building their own engines i.e. the 6.7 scorpion. I've driven quite a few of them looking at trucks the last year or so and I have yet to find one that I think could honestly hang with my 6.0 without major mods and tons of money poured into it. I just wish the 6.0 didn't have such a crappy reputation I've had some problems with mine but it's got 175k miles on it and there was a lot of towing going on for most of those miles. Problems are expected.