The main issues are the location / position of the Turbo and the radiator / cooling.
The Turbo on the Discovery engine is on the side of the exhaust manifold and as such routing the intercooler pipes / exhaust is a bit of a chore, but not impossible. The Defender has it mounted in a similar position to the TD.
The intercooler / rad itself is different too, and you would need to either source a Defender unit, or fit an after market intercooler.
The job is possible, but it's not the 'drop-in' that a Defender engine is. You will likely find that you will be a couple of hundred quid short between selling a Disco Tdi and buying a Defender Tdi for this reason.
If you are confident to do a bit of fabrication work, and not afraid of a bit of problem-solving, then stick with the Disco engine. If the prospect of that scares you... go for the Defender