A few years ago, the usual upgrade from a 2.5NA was to a 200 or 300tdi from a rusty Disco. Which is best is the landy equivalent of which calibre is best - both are broadly similar in terms of power and economy but each have individual characteristics that endear them to some. The Tdi's are still the easiest way to go as the conversion is pretty simple and relatively cheap. If you look online, you will find many many step by step guides with photos. However, the supply of cheap 200tdi's is drying up fast, and I would expect the 300tdi's will be too - Landrover have stopped making rusty Disco's and many people have converted to use (and abuse) them.
The Td5 was shunned for a long while as being powered by black magic (such as an ECU) but is now accepted as a basically good engine (at least the post 2002 Storm 2 version) with slightly better power/economy and much greater potential for tuning (see the Twisted Performance website for comparisons). The conversion is considerably more difficult and parts are more expensive.
I would strongly recommend not even thinking about a Puma conversion - it is very difficult, expensive, and (in practice) not enough of an improvement over the Td5 as a conversion.
Which should you get? if most of your driving is offroad or relatively low (mostly 60ish) speed, and if you want the least expensive conversion route and subsequent maintenance and repair, go for a Tdi. If a significant part of your driving is faster (70+) or if you want something quicker and smoother, go for a Td5.
Regardless of which you go for, preparation can make an enormous difference to the ease of conversion and subsequent running. Hop over to
Land Rover Owner Index page for all the advice you could ever want or PM me.
Knots