Don't worry about what it looks like and spray booths etc, watch out for what's underneath!
In 1983 Land rover stopped the series vehicle and started making what we now know as defenders, but they called them 90s and 110s (for long wheelbase). In diesel models, these had a 2.5 litre engine, na (non turbo).
A few years later there were lots of complaints about how weedy these engines were so they bolted a turbo (more a turdo really) to it. This is the 2.5td. These engines were a compromise and didn't really cope too well with the light turbocharging, although i must say mine never gave me any problems. It also never gave me any power, which was a bit of a problem i suppose, and it was a bit thirsty.
In 1990 they re-branded the 90 as a Defender and put in a complete new engine, the TDi (initially the 200). This was a cracking engine, designed for more power and economy, and it delivered too. Incredibly reliable, lots more power, easy to work on. EU emmissions legislation meant in about 1994 they had to make some amendments, and it became the 300, but was essentially the same engine. The 200 easily fits in the older 90s as a conversion. Discos have virtually the same 200 and fit well in the old 90s.
The difference between the td and the TDi can easily be a grand, even though the cost of conversion can be less.
Hope that helps!
OD