I changed my 2.5 10v petrol to the 2.5 TDI some years ago and, as said, you'll need to drop all the front running gear, exhaust, fuel lines, fuel tank, engine loom, instrument panel, ignition and immobiliser. The cabin wiring harness can stay intact and just feed the TDI engine harness back through the fuse box aperture and connect the multiplugs. Sounds like a lot of work but if you have suitable space and a rattle gun for disassembly in you armour you could achieve most of the heavy work over a long weekend.
Provided the pics are still available to view you can have a look at my
Project TDI thread. Admittedly it's been a few years since I updated the thread (lost interest after the PhotoBucket debacle) but if you need any other info or pics must let me know. I keep meaning to start a new thread and embed the photos to this site.