A couple thoughts:
If you're happy with your swb, look for ways to make it a more capable tow rig. swapping in a 2.8 sounds like a good start - can you increase the boost for more power? Adding a less restrictive exhaust system seems to be a very effective modification on most turbo diesels.

Towing a heavier trailer might mean stiffer springs or air bags to help with the extra load on the rear supension.

Balance the cost of making your existing Montero into what you need vs. buying another vehicle & I'm betting it'll cost less to improve yours.

The biggest disadvantage that I see in keeping yours is the short wheelbase. My experiences in towing have convinced me that longer wheelbase vehicles are much more stable when towing, although the heaviness of the Montero will help offset the shortness.