Diesel vs. petrol economy - flunky
I need to buy a pick-up truck - in Indonesia.

I'm not going to be driving it.

It's going to be a Mitsubishi Colt, either a T120SS or an L300.

The options are:

L300 - Mitsubishi Astron 4D56 diesel engine 2.5 litre - 74PS @ 4200rpm, 147N torque @ 2500rpm. 2500 kg gross vehicle weight (1375kg maximum cargo weight). Cargo bed 1.6 x 2.425 m.
www.ktb.co.id/product_detail.php?kid=8&pid=44&st=6
www.ktb.co.id/pdf/43.pdf
Cost new: £9,100 (a 5-year-old example costs £5,600, and a 12-year old model £3,400)

Or
T120SS - Mitsubishi Orion MPI 4G15 petrol engine 1.5 litre - 81PS@6000rpm, 115N torque @ 3500rpm. Cargo bed choice of 1.48x2.2m or 1.6 x 2.335m. Gross vehicle weight 1760 kg (970 (smaller cargo bed)/820 kg (larger cargo bed) max cargo weight).
www.ktb.co.id/product_detail.php?kid=7&pid=54&st=6
www.ktb.co.id/pdf/54.pdf

Cost new: £5,600

It would be used to carry large loads of vegetables

The cost of petrol is £0.39/litre, and diesel £0.36/litre. Although this might seem cheap by British standards, fuel costs would actually form a significant part of running costs.

Typical load would be a 60 mile journey laden with a few hundreds of kilos of veg passing through various villages (plenty of braking to avoid potholes,goats,etc.). Any thoughts on how the petrol 1.5 would compare with the diesel 2.5 on running costs? Both engines I believe are pretty primitive jobs.
Diesel vs. petrol economy - flunky
another consideration is durability - as shown from the used prices, depreciation is very low and a vehicle would be expected to last 20+ years.