There are a few threads where this has been discussed, but I am having trouble finding them right now. I will post a link here when I do.
In general they are solid vehicles with great capability. They did have terrible depreciation, so later owners may not have taken the best care of them. Gen 3 (01-06) is the most recent that was sold in the US. That 03 should have a 5-speed transmission and an electronic shift 2-speed transfer case with 2H, 4H(AWD), 4HLc (4WD), and 4LLc (4WD Lo) on the shifter. It should also have the 3.8L V6 engine which makes a bit more power and doesn't have as many minor issues as the 3.5L. Check for a throttle cable (3.5L) or a drive-by-wire throttle (3.8L) to confirm. Also if there is a ASC disable button in front of the shifter, then it is a 3.8L as throttle cables cannot have stability control. Look for slight oil leakage around the valve covers. This is common on both versions, but will eventually need replacement with anything other than Fel-Pro (poor fitment). With good care, these seem to last 200k+, but they will need some maintenance around 165k to get them past 200k.
Look for coolant in the valley under the intake manifold. There is an o-ring in the back of the water pump that mechanics may not remember to change. This will eventually cause overheating. The part is cheap, but it takes a while to get to the back of the pump for the repair unless you are already doing a timing job.
When shifting into 4H, the wheel indication lights should blink then engage. This can be done while driving in a straight line under 60mph. This does not lock the center diff and acts as an AWD mode for slippery pavement. Other modes with 4HLc (Locked center differential) should only be shifted while stopped. 4LLc requires N while stopped. Both locked center modes should NOT be used on dry or wet pavement. You may need to release the brake or shift to D/R to get the system to align slightly. It should all go back in reverse order.
I have used both versions of the Gen3 off-road on the East Coast, and there isn't much it won't handle. Stock vehicles will fit 285/75R16 (33") without rubbing or modifications. The IFS/IRS setup drives differently than a solid axle, but I have found it more comfortable and balanced over obstacles. Lifts can be found from Old Man Emu, King, Bilstein and others. They usually gain 40mm (~2.5"). As this is a suspension lift, that also creates more ground clearance without even increasing tire size. They should fit 34-35" with a 40mm lift. There is no body lift available.
All Pajeros/Monteros were designed to allow the seats to fold flat into a sleeping platform. It is not exactly a flat mattress, but it would work until you can make a sleeping platform.
I use mine for towing as well. There are several hitch models available, but they do hang low and should be watched off-road. I have a Skidmark 4x4 hitch plate to use as a guard for my trailer wiring and to support the back end over obstacles.
Aftermarket specific support is minimal in the US. Biggest market is Australia for these, but some items can be ordered through US vendors.
You can browse the museum link in my signature for some more information on the vehicle and its history. I have another for the Factory Service Manual downloads. Grab one for the 2003 model year. It will have some sections for diagnosing and identifying issues.
