alright here we go.
gm.
2001-2004.5=lb7
injector issues.
not if, but when. soem are good for a long time... injectors leak and fill the crank case full of fuel. not a fun job to fix but doable- injectors are inside of the valvecovers and require a special tool to remove, additionally the injectors are 300+ ish each.
they make good power and get good mileage, and most of the "fast" duramaxes are these motors.
2004.5-2005=lly
5 speed allison just like previous years... engines got the injectors outsdie of the valve covers... have a thing with overheating tho... which is fixable with different "mouthpiece"
2006=lbz or lly- mechanically the same as lb7- with 6 speed allison but not the same gnarly tune as a 07 lbz which was only available in the nbs (old body style for 2007) strucks.
6 speed allison is sweet and overheating issue was figured out
those are about the only gm's youll find around 15k possibly.
ford.
7.3. nutswingers like a gtr at an import night. they ended in 03.5... some 03s have em- lots have the 6.0
decent mileage... i drove a 7.3 excursion to wyoming and got 18mpg going 78mph... if this isn't decent mileage i don't know what is... can't be made very quick very easily- unlike the 6.0/cummins/duramax. easier to go faster in other diesels.
6.0 03.5-05... this engine is why the 7.3 is famous. egr delete is necessary imo on any 6.0... the egr is cooled by coolant from the engine, when it passes through the hot egr, it superheats the coolant and thats why you hear about popped head gaskets... i personally don't know what got better in the next gen 6.0 and you would have to find out to quote me, but many people like the newer 6.0s quite a bit. can be made quick with a tuner and get decent mileage. and sound o so sweet with exhaust....
05-07 6.0s. many lovers. many haters. personally if was looking for a diesel truck- this is exactly where i would be. the 6.0s are quick with tuners. additionally decent mileage. i would personally also delete the egr, headstud it, and put a tuner on it and fuck shit up. the ford trucks feel so much more truckish than the gms do as well. whether that be your thing or not. ifs rides so much nicer than the straight axle stuff imo...
dodge i don't know shit about. i like em tho.
wish i could shine some light with some knowledge behind it unlike spitballer boy above just blabbing.
so there you have it... if i were in your position- i would personally be looking for a late model 6.0... the injector thing scares me on the duramaxes and to get into a decent 04/05 duramax is normally big money. since there are so many 6.0s around, they seem to be cheaper imo than their duramax counter parts.