Welcome to the Wire,
What you have are Gen3. All the versions before yours are body on frame with a solid rear axle on trailing arms, well Leaf springs for the Gen1 4cyl that we first got in the early '80s. The front suspension is double arm with torsion bars, steering is old school recirculating ball rather than the rack & Pinion as you have. Basically, they are trucks.

As you know your two early ones have a 3.5L engine, 4 speed auto with 4.9 gear ratio.
Your 2003 has the 3.8L, 5 speed auto and 4.3 gear ratio.

Parts from the previous Monteros or it's cousin the Montero Sport have very little compatibility.

In the Gen3 there was a period with failing head gaskets with the 3.5 engine, that supposedly was cured on the 3.8, but then the intake has "butterfly valves" which are prone to failing and have a rather long history of doing so. Just removing the vacuum line from the actuator will disable them to the open position and all but eliminate the possibility of failure.
The swap to the 4th gen engine will require a wire harness change as well, not necessarily easy for most people. Not to mention compatibility with US emission test might not be easy.

In country engines, the car 3.8L engines have higher compression and provide more power. The engine block has some differences where the mount brackets attach which with the alloy brackets on the Gen3 may be a bit frustrating since not all bolt positions are cast into the block. It can be done though. The factory ECU can be programmed as needed but not sure how hard to find a tuner.