The pickup models from the mid 90's to early 2000's were new enough to suffer lest rust than predecessors, but old enough to still offer simple (less powerful) diesel engines with mechanical injection and no turbo. These were longer lasting and cheaper to maintian for farm / local use. E.G. 6th gen Hilux / 3rd gen Triton.
The next generation included common rail fuel systems and turbo twin cam diesel engines. Auto transmissions and ABS became more common on high trim levels and crash safety improved considerably. These perform much better on the highway, but the common rail system and turbo dramatically increase engine rebuild cost, when the time comes.
Toyota Hilux, Isuzu Dmax, Mitsubishi Trition and Ford ranger as above, all seemed pretty solid to me. They are all reliable, if not abused or worn out. The Hilux and Dmax generally sell for higher prices, Mitsu and Ford being better value. I have less experience with the Nissan Navara, Chev Colorado, and Mazda BT50. The BT50 should be similar to the Ranger, Colorado similar to Dmax. With vehicles of this age, the biggest difference will be related to their condition and maintenance.