3 Liter 5 Door Pajero is def under powered car as compare to other cars. Simply use body to weight ratio: 2Ton on 170 HP is 85hp/ton is sluggish in pickup.
However to improve it, get the full service done and change the spark plug to platinum plug, change air filter to sports filter like (K&N) and see the tad difference in acceleration.
If the gear oil and filter is new, then i suggest disconnect battery for 5 mins and see if it helps.
If not you definitely need a gear expert to check the gearbox.
Drive Chain or Timing best is essentially changed between 75.000 Km to 100,000Km. so yes my guess is that he is right but if you are still not convinced then it is better to take and advise from another mechanic.
IT is the summers my friend. tiers are the first to go in the car, it can be that the air inside the tier is expanding that is causing minor leaks and if you don't get alarmed now you may get into serious trouble so change the tier now
If you or the previous owner didn't change the fuel filter as a preventive maintenance then it is possible that your fuel pump is choked with all the fuel impurities.
You must be looking at very high mileage engine throwing black smoke at start up. You can try engine clean up if helps in de-carbonize engine (25% chances), if not this calls a sure shot rebuild of motor with quality parts.
Engine oil usually change from 3000 kms to 5000km if you serviced with mineral oil. If fully sythetic oil then it should last 8000kms 10000 kms.
Oil has life per km not months, but if you drive too much short distance runs then its advisable to change it more frequent than oil manufacturer recommendation. If its dark, better change it before it do more harm. Oil is cheap and cost much less than one fuel tank cost here.
Al sheeba trading is distributor of King springs and shock are best in market with very good value for money and performance.
They in aweer area, google their number.