Older (uptill 1997) model Mini Pajero 660cc came with 3-speed Auto Transmission and a clutch type radiator fan.
With 3-speed transmission, if you go above 80Km/H, the engine will be at around 4000-4500 RPM, too much for a tiny engine for longer runs. If you go till 100 Km/H, it will rev at 5000-5500. So for a short time that's OK, but for continued drive, it will be very tough for the engine and your ears (lot of engine noise).
Solution, buy a newer model with 4-speed A/T, or use it around 60-70 Km/H.
About heatup issue, after 23 years (the car age) the clutch oil in the fan clutch is either leaked or have lost all it's thermal properties. As a result, at low RPM the engine doesn't get cooled enough and more heat is produced by the use of AC.
Solution: Get the radiator changed with Electric Fan type radiator, heat up issues with AC will be solved.
Slow speed drive of several hundred KMs is no issue, I just made a trip to Skardu-Khaplu via Babusar - Deosai on 660cc Auto Mini Pajero without any issue. But the average speed was much lower than on motorway.