Hi,
The jonga has a 6 cyl 4 lieter powerplant producing 145hp and 32.5kgm torque(with the hitarchi carb) and 135hp and 30 kgm(with solex carb).
The diesel version produced 110hp on a hino 4 cyl.
If you are talking about relaiblity, the Jonga is more relaible than the Jeep petrol(hurricane motor), but as far as diesel is concerned, the Di Jeep is much more reliable.Leaving that aside, the jonga has a much more tough and relaible drivetrain than what the mahindra stable can ever produce(even the modern ones).
Reliablity issues are more to do with maintainence, when you dont retighten things when you are supposed to,, like when both the high low and 4x4 engagement levers fell off my moving Willys.
I am currently inbetween my project of a heavily modified jonga,with a thicker axle shaft to run 33 boggers and a cummins 6bt 250hp motor, a four speed low ratio (7.13)gearbox with a pto winch and a lot more stuff. And whenever i went looking for parts, i never ever found new parts(except seals and stuff). The only place you can find parts is mayapuri, where you'll get army hand me downs ,but you need to have an eagles eyes to get something worthwhile coz most good parts that could be reused have been degraded just coz they have been lying around for so long that rust eats them right through the grease layer.
If you want a good vehicle, my advise would be to stay off the jonga ,even though it is a much better option, coz you wont find parts. And even though you can change the entire drive-train and put dana axles and stuff, the body parts are still difficult to locate(but that i think goes for the jeep too, i just cannot find stock front mudgaurds anymore, not the classic ones but the originals). The jeep petrol parts too are difficult to locate, but the diesel parts are easily found. Not that the jeep unit is pretty sound too, the DI/inter motor sits too low on the chassis, keeping it in constant danger of damaging the chamber in rocky offroad conditions(happened to me twice). The 3 speed gearbox, though good enough,prohibits you to use a high ratio final drive, and if you assemble it with a turbo Di, you are more likely to have broken parts ever now and then(cannot handle the torque of a turbo dI).The 4 speeder is better, but you should overhaul it with new bearing coz its the only problem it has(besides having thin gears, but that would only trouble you if you have an engine that produces more than 28kgm torque). Axles and brakes should do fine enough if you upgrade to a Brake Booster with dual master cylinder, and classic's disc brakes. If you go with a full-floating rear axle, than you can even shoehorn rear disc brakes to it.
And yes, whoever said that you can use engine brakes in a diesel motor is wrong, coz unlike petrol motors, which control the air quantity entering the engine to produce more power and speed, a diesel motor has a constant supply of air flow and it controls power by the amount of fuel injected in it. A petrol motor can retard engine speed down a hill by just lifting your foot from the accelerator pedel, coz it closes the bullerfly/slide valve restricting the amout of air entering the engine, henve creating a sort of vaccum inside the cylinders, therefore slowing the motor, whereas in the diesel motor, the air entering the motor is not controlled(remember its controlled my fuel amout ,not air) ,so going downhill will only speed up the engine. That why heavy duty diesel motors have a exhaust brakes, to slow the vehicle when going downhill. An exhaust brake is nothing but aa butterfly valve, but used in the exhaust manifold. Closeing that valve restricts the amount to air exiting the cylinder and creating backpressure, hence having the same effect as in a petrol motor.

Regards,
Bikram