I owned a 1994 Rodeo 2WD and worked at an Isuzu dealer for 5 years. I also had a 1991 Amigo XS 4x4.
First: The Amigos through 1995 were all 4 bangers, underpowered and got bad mileage. The second gen 1998-2004 were much more comfortable and got better mileage, even with the V6. Do NOT consider a 4 banger second gen Rodeo or Amigo, that Holden motor is crap. All 1998-2004 Amigos can be considered lumped in with the Rodeo info below.
Rodeos from 1991 to early 1993 had the 3.1 GM V6 and were underpowered. The Isuzu SOHC 3.2 V6 came out in 1993 and was a vast improvement. That engine was used with minor changes until 1998, when the DOHC 3.2 replaced it. All from 1991-1997 were available with the 2.6 4 banger, same as the Amigo. It's tough but underpowered and was not available with an automatic, or at least I never saw one with a slushbox. 1993 3.2's were known for oil leaks, 1994 and up were pretty oil tight. 1995 1/2 saw the introduction of the black 'Honda dash' as we called it and had OBDII. That continued to the end of 1997. 94-97 are going to be the ones at the low end of the price scale.
Overall, they are pretty tough vehicles and will tow 5000 pounds with an automatic. Their Achilles heel is indifferent maintenance (lack of oil changes) which causes the rocker shafts to wear, making for a pretty nasty clatter. If you go look at one, first pop the oil fill cap off and look inside the valve cover. If it's gold or even a very pale reddish color inside the valve cover, chances are you are OK. If it's dark red or black, that's a sign of poor maintenance; keep looking.
The transmissions are GM 4L30's, they are actually pretty tough. I saw very few (to the point of vanishingly small) transmission complaints. The axles were made by Showa and I never saw a busted one. In fact, I saw only 1 Showa axle with a problem the whole time I was there: a broken spider gear pin in a Trooper. But, they do like to sing to themselves.
The 94-97 V6's all had 4 wheel disc, 4 bangers had disc/drum. You will hear they break wheel studs very easily. That's because the torque spec is 65 foot pounds, overtorquing pinches the lug nut in and the stud breaks off the next time you try to remove it.
98-up had the 3.2 DOHC V6. I saw a few very early 98's crack heads. Otherwise, they were pretty tough. 2WD versions had disc/drum, 4WD had 4 wheel disc brakes. Isuzu dropped the Showa rear axle in favor of a Dana unit, I saw several of those develop bearing problems.
Towards the end of 2002, there was a running design change on the 4L30 transmission and a servo cover could pop out, causing the transmission to empty itself very quickly. Those have probably all been fixed by now. In 2003, Isuzu went to the Aisin Warner transmission (same as the so called Toyota transmission in the little Cherokees) and that was used till the end of production.
The wiper delay module on the 98-up trucks could puke, meaning the wipers had no delay or intermittent function and would not park. The module is at the top of the right kick panel and is light brown in color, easily replaced.