If diesel it is you're gonna put in your rig, you already have recommendations from various experienced users. Here're my two cents.
Why go for a diesel, when you have patrol alternatives. I'd say go for a decent Toyota Patrol Engine, a wide variety available in Shershah Market, ranging from 3Rz to Vz, and the legendary UFz and FZ engines 4000 and 4500 cc respectively. The two later produce from staggering 190-225 hp at 4500 rpm more than enough to quench the offroading thirst. The price tag, i'm not quite aware of.
Since you'll be paying for the engine and labour, i'd say, think thorough and make your mind.