Hello,
First you need to know that Pajero Mini is really a 4x4 vehicle, and its gear ratios are designed for off-road and torque, not speed.
The car you currently own is for speed and comfort, and it gives you pretty good mileage. Pajero Mini 2012 models are pretty good ones, but due to its gear ratios, you'll be getting max 12km/L average, and nothing you can do to improve it.
If you are okay with bit expensive running costs, parts and other spares are not a big deal. Every big city have them.
And lastly, yes you can take it to 100km avg speeds on highways and do inter-city travel without any issues.
Also read my reply here https://www.pakwheels.com/forums/t/looking-to-buy-suzuki-potohar-price-range-400k-is-it-a-good-first-4x4/258144/44
Look for one not touched by local mechanics, and keep it maintained with good spares and oils, it'll serve you good. Good luck!