Check out this post for a lot of good info on debunking some of the myths of the truck; http://discoweb.org/forums/showthread.php?t=46593
1) CDL -- 99/00/some 2001s have the right transfer case, but no linkage, you must either get a 2004 linkage, retrofit a d1 linkage, or do something custom... 2002/2003 does not have the 'nipple' on the transfer case to engage cdl.. either need to modify the transfer case, or do a swap.. 2004 has the cdl linkage
2) it'll rust similar to a D1... i dont notice any rust in the door jams tho like in the d1s
3) headgaskets do seem to go often, but basically when you replace it once with the updated gasket, you're good to go
4) i'd personally stick with coils, no SLS to worry about
5) fuel economy varies depending on modifications and driving habits.. if you keep it stock and have a light foot, you could probably see 16.. i usually see 9city/12hwy, but im on 33s and lots of weight in mods.
If possible, go with a 2004, they are the best of the bunch, 4.6 engine, CDL handle, and just a lot of things 'fixed' from previous years.
Be wary of some 2003s, as they have a recall on oil pump failures that take the whole engine with it.