The '04 was the last year for leaf springs in the front, the '05 up are radius arm/coil springs. I personally think that the 05 up ride a little better, it's a little smoother going down the road anyway, but it isn't that much of a difference.
Not sure what the cost would be to get 35s under either one though, mine is still stock height and 33s should fit without issue. I think that coil spring spacers and F350 blocks out back would most likely clear 35s on the 05 and up F250s and spacers alone on the F350s.
I'm fairly sure that the leather seats were Lariat only issue, unless someone either swapped them in or put aftermarket leather covers on the seats. Either way, I doubt that you would have too hard of a time finding someone who would trade with you.
Another option, and I forget now who did it, but there is a write up here on this board where a guy had factory captians w/ console, removed the console and the arm rests and then installed the "40" part...just have to do a little hunting for the artical.
As far as power, I'm happy with my 6.0L. Add a few upgrades to it and it will really scoot. They aren't really any louder than the 7.3s I don't think...but the Duramax trucks I've been around are super quiet. I've got a 4" MBRP on mine and it is a little louder, but not much until you open the throttle...and even then it's not stupid.
