Welcome Dave!As far as maintenance goes, it definitly isnt more than gas vehicles. A lot of people change oil less frequently with diesels. The Powerstroke, if you take care of it will last you a lot longer than any gas engine.
Of course, it might depend if you are going to be doing a lot of towing, but if you are going for fuel economy, you might want to get something with a 3:73 gear ratio. Stay away from low ratio's like 4:10's.
As far as 94-98's are concerned, I would look for a manual transmission. Ford did have a goot bit of problems with a lot of their automatic transmissions in those years. However, if you did get an automatic, a good tranny guy that knows Ford problems can fix it so you never have to mess with it again.
Depending on the truck, gear ratio, how you drive, etc you could get anywhere from 16-19MPG. With a few mods you could maybe get more. No way of telling till you test it though. Every truck is different.
Hope this helped!