If highway usage is important, forget about the VW diesel - small diesels in my experience (and so it's clear, this is general small diesel experience, not specific to the VW/Samurai conversion), are woefully underpowered, and produce that power at low rpms - gobs of low down torque, but limited horse power. Turbo diesels are better but still sub par when compared to a gasolene engine of similar displacement, and this is on a vehicle fitted with a diesel from the factory, so presumably the gearing is optimized, which will not be the case in a VW/Samurai conversion.
If what you want is low down torque for crawling up rocks, the diesel is probably your best choice, but do not expect to get anything like decent highway performance out of it.
Comparing my 2.0 Grand Vitara to a naturally aspirated Mitsubishi 2.3 diesel L300, and a 2.3 turbo diesel L200 - the L300 is sluggish and an absolute nightmare when overtaking on a two lane black top (one lane in each direction, no median), the turbo diesel L200 is a distinct improvement on the L300, but still no match for the 2.0 GV.
Consider either a 1.6 G16 swap, which is relatively common and kits are available, or even the 2.0 J20, which is less common, there used to be kits available for this also, I'm not certain if they are still offered.
