Well, the independent front suspension that was used on those trucks worked well. The only trouble with them that I know of is alignment...good luck. The 6.9 is still available from a few places as an exchange crate engine usually around $3500 for the complete engine (minus alt,a/c, pwr steering etc) however I would replace it with a 7.3 IDI non-turbo, or if you get lucky you might find a 7.3 IDI with a turbo ford did make them as well, just weren't very popular around the eastern end of the US anyway. As for fuel consumption on the 6.9 I am not sure...my Father still runs a 1993 F-350 with a 7.3 IDI non-turbo and is getting around 17 miles per gallon with an overdrive auto tranny. My guess would be around 15mpg or so with the C-6.
Hope this may have helped a little I am sure there are plenty of others on here with more useful info. Good luck with the restore. post some pics when u can.