FatTony, that is a broad range of years that will get you many differences in truck, and I think you need to narrow it down more.
'94-98 12V P7100 Injection Pump. 47RE 4spd Auto or NV4500 5spd Man. Quad cab got the 4 doors in 98 (only year that the 12V got the 4 doors). CAD (Central Axle Disconnect) on front axle with vacuum actuator.
'98.5-02 24V VP44 Electronic Injection Pump. 47RE or NV4500 5spd Man, IIRC it was '00.5 that you could get the High Output rated at 505ft-lb and was only had with the NV5600 6spd manual. Also has the CAD axle vacuum actuated. Exterior remains same as '94-98 trucks with minor differences, interior changes easily noticeable. Common problems are failure of the engine block mounted lift pumps, which will cause the injection pump to fail. Rule of thumb is to maintain 5psi fuel pressure to the IP for lube/cooling. New IP is ~$1200 last time I replaced one. #53 engine blocks prone to cracking, as old as they are now any that was going to crack would have already done so, so I wouldn't worry about this. I think they got disc brakes on the rear axle in '00.
'03-04 24V Common Rail CP3 Injection. 47RE, 48RE, NV5600. There were 3 different power output levels available, Cali, Normal, HO. Doesn't matter to the aftermarket as the tuners/chips all work the same. 2 Injection events. Loss of CAD axle, it is always live, the front drive shaft is always active. HE341 turbo. The later model HE351 is a direct bolt on. 4 real doors, but the back seat doesn't have much more room than the earlier model trucks, but it is more useable. Problems are high pressure injectors require good fuel filtration; don't skimp on filters, and many people add external 2 micron fuel filters to increase injector life. If an injector fails open it can torch the piston.
'04.5-05 24V Common Rail CP3 Injection. 48RE, NV5600. '05 might have been the first year of the G56 6spd man that replaced the NV5600, I can't remember, but the G56 has a shorter OD ratio, and the 48RE has the tallest OD ratio of the group. Bump to 325HP. Engines gain 3rd injection event (ever so slight loss in fuel economy) for emissions reasons. HE351 turbo. All trucks are considered HO and have same power rating regardless of location or transmission. Injectors have different spray pattern than 03-04. Problems are high pressure injectors require good fuel filtration; don't skimp on filters, and many people add external 2 micron fuel filters to increase injector life. If an injector fails open it can torch the piston.
'06-07 24V Common Rail CP3 Injection. Still 325HP. 48RE, G56. Update to front clip and complete interior. MegaCab available. Different engine computer allows for EFILive tuning (more memory). All electrics go to TIPM (Totally Integrated Power Module) which is a smart relay control module thingamajig that likes to shit the bed on a short circuit or overload condition. Problems are high pressure injectors require good fuel filtration; don't skimp on filters, and many people add external 2 micron fuel filters to increase injector life. If an injector fails open it can torch the piston.
'07.5 First year of the 6.7L (loaded with emissions garb), 68RFE.
May dad has an '01 (278k, Edge-EZ module), my FIL has a '99 (stock), I have an '05 (Edge Juice/CTS and shift kit), and my dad has a '14. When I got my '05, we were all amazed in how much more comfortable, quieter, smoother riding, and more power it had over the '01. We now say the same thing with the '14 when compared to my '05, and it makes the '01 sound and feel like a diesel farm tractor from the 80's.
All have held their value really well in the used market, and it is hard to find a clean one anymore. Manual trans are going for a premium, it's insane.
Tough choice on which one to get. All will be similarly capable offroad because they are so similar on the axles & suspensions. If I had to choose, I'd get a 2nd Gen (98.5-02). Less electronics less wires, easily converted for 2-Low operation if desired. Feels like heavier/thicker body than the 3rd Gen trucks. Injectors are much cheaper than on the CR engines. But, if there are more than 2 people going, then get an '03+ for the extra leg room.