1989-1993 was the first generation of Cummins engines in 3/4 ton and 1 ton trucks.(W models were 4wd and the D models were 2wd) They were equipped with a VE injection pump. Most inner-cooled models produced around 160hp and 400 torque from the factory.
I personally own a 92 W250 extended cab and love it to death, I won't ever get rid of this truck. 230k miles and running strong, no leaks everything original.
1994-1998.5 was considered the second generation of Cummins equipped trucks. Both 3/4 ton and 1 ton trucks came equipped with 12v p-pump/p-7100 injection pump Cummins engine. This generation is considered the most powerful and just as reliable (if not more) than the first generation. 94-95 produced around 175hp and 420 torque, and the 96-98.5 produced about 5 more hp. The manual models produce about 35 more hp and 20 more torque.
Both generations are held up with the same problem...Automatic transmissions from the factory. A518 from the first generation and the 46RH and 47RH from the second generation. Both Transmissions can be made bullet-proof with a rebuild.
Both generations should have the KDP (killer dowel pin) tabbed if they haven't already been. The KDP was more of a problem in the second generations.
Also some 98-98.5 models have a number 53 block which has been reported to crack. Not sure if all models with the 53 block will, but better safe than sorry.
Single cabs are a dime a dozen in both generations of all-mechanical Cummins engine equipped trucks. Extended cabs are the real jewels to find in good condition. And I think there were maybe six 4 door versions built in the first generations....good luck finding one of those.
Both generations are awesome trucks, but its all personal preference. Both make lots of power real easy!
To really learn some more detailed info go to the forums, such as Cumminsforum.com and dieseltruckresource.com. Also you can check out dodgeram.org.
Good luck in your search! You won't regret buying a cummins, just hold out be patient and find the one that you want don't settle.