The auto's are weird, there is no rhyme or reason why they go out. Some go out behind stock powered trucks at 55k (like mine) or go 250k?! It really is determined by how you drive, aftermarket stuff and luck.
An aftermarket built auto will cost in excess of 2500 usually more if including a T/C which is usually over a grand itself...
The auto will be a bit faster then a manual simply because the shifts are much quicker. If you plan on racing it, I'd get the auto. For sled pulling you can use a manual just as well as an automagic. For a lift with oversize tires the manual might be better for you on the hwy but generally I like the auto better just because I talk on the cell, drink my mountain dew and don't have to jam gears anymore (like I did in my old truck)...
As for the price, you should be able to find a late 99 or 00 model Crewcab, lariat 4x4, 7.3 with around 100k under 20k. The reason I say late 99 to 00 is because they came with forged rods, which hold up better to power then the PMR's that came in the 01-03 trucks. If a chip is the only poweradder you want then any year from 99-03 will do fine.