1. What does general maintenance alone cost per mile (mostly oil changes I guess).
Assuming you do most of the simple stuff yourself, then the main difference in a diesel and a gasser is the extra oil you need, the higher price for the oil filter, and the occasional cost of a fuel filter/water seperator.
The engine holds 15 quarts of oil, counting refilling the new filter. Most big-engine gassers hold 5 or 6 quarts. So go to your oil retailer and price diesel-rated motor oil, such as Shell Rotella T or Chevron DELO 400, and figure 10 extra quarts. The huge oil filter is about twice the cost of the smaller filter for a gasser. And the fuel filter is needed only once ever 15,000 to 30,000 miles, but they cost about $30 each.
I have been told oil changes are every 5000 miles.
Your oil change interval (OCI) will vary a lot, depending on your exact climate and driving conditions. If you're smart, you'll use oil analysis to determine your customized OCI. So that's an extra cost up front of about $23 per oil change, but only for the first three OCIs, then after that you can do the oil analysis only about every third or fourth oil change to verify that your OCI hasn't changed.
My OCI is 5,500 to 6,000 miles, but almost everyone else with a '99-up 7.3L has an OCI of 7,000 to 10,000 miles. So if you don't live in a dust bowl, and you don't constantly tow a heavy trailer, and you usually allow the Dodge to win the stop light grand prix, then your OCI will probably be closer to 7,500 miles than to 5,000.
But use 5,000 miles as your OCI until Blackstone Labs advises you differently.
2. What kind of reasonable MPG can I expect on the highway/city?
Varies all over the board. Lots of folks claim over 15 MPG, and some over 20 MPG. Mine is about as bad as it gets, at about 14 unloaded and mixed drivingconditions and 12 when grossing 16k with a mid-profile 5er and holding down the speed to 1,800 RPM (62 MPH). On a long-distance highway run at 74 MPH I get about 15.5. But if I have to rush when towing, mine will get about 9 at 74 MPH (2,150 RPM).
I get a chuckle out of some dude cruising at 74 MPH with a PowerStroke draging a high-profile 13-foot-tall 5er and keeping up with the big boys at 74 MPH. Or out west of here a few miles, the speed limit on I-20 and I-10 is 80 MPH, and those dudes cruise at 84 MPH with that tall 5er. Yes, the PowerStroke will haul the mail, but you're going to pay for it at the Flyin' J. So if you're that dude, then don't count on more than 8 MPG between Odessa and El Paso.
As a general rule, for cost benefit analysis purposes, figure 20 percent better fuel mileage with a diesel than with a big-block gasser of similar towing capability. Of course, there is no such thing as "similar towing capability". The V-10 Ford gasser will struggle to tow a heavy trailer that the diesel would laugh at.
You can see the difference in cost per gallon of diesel vs. gas right now at any truckstop. But understand that until a couple of years ago, diesel usually cost about a dime a gallon less
than regular unleaded gasoline. So you're going to have to make some educated guesses at the cost differential in the future.
Over nine years ago I did the cost/benefit analysis on my truck. It showed that the diesel would be less expensive in the long run - after around 100,000 miles. That was over 177,000 miles ago, so I'm glad I paid the extra up front to get the diesel.
And that's not assigning any value to the pleasure of towing my 5er up a mountain pass and passing all the big-block gassers on the road. The only competition is the occasional Cummins.
Yeah, the V-10 Ford will probably get you there - eventually. But when you're climbing from Denver to the Eisenhower tunnel and several diesels pass you, you'll probably wish you had assigned a bigger price to that mountain climbing difference.