Anything gas is probably ok. From 2002 1/2 forward stay away from the diesels. The 6.0 is crap and the 6.4 isn't much better. yes there are some good ones, but if it's a good one you most likely won't find it used.
I have known several people who had the 6.0 and 6.4 that had nothing but trouble with them and ended up getting rid of them.
If you find a 7.3 in good shape used, grab it.
"Perhaps the final test of anybody's love of dogs is their willingness to permit them to make a camping ground of the bed" -Henry T. Merwin
I purchased a new Ford F-350 diesel dually in 2004. They re-designed the diesel engine in 2005 models and then again in 2010/2011. I have had good luck with my truck, although I hear that the 2000-2004 diesel models may need to have head gaskets replaced when mileage gets 70,000+.
I plan to trade for 2011 F-350 diesel dually within the next few months.
Just an update - I bought my "new" truck today. I got a 2002 w/133,000 miles (I have to keep reminding myself that this is just getting broken in for a diesel). Best thing? I was working with an insurance payout from my previous truck which was totaled in an accident, and I was able to buy it outright with enough cash left over to put in a bed liner and a gooseneck hitch and upgrade the brake box.
Thanks everyone for the help (and JB for referencing the previous thread).