I have just sold my Defender double cab TD5 after owning it for 14 years. It was a cracking machine, but I never managed to cure the leak into the passenger floor from somewhere around the windscreen :-(

I put a new high ratio transfer box into it (Ashcroft's with Discovery quiet gears), a new gearbox(Ashcroft's again, well worth buying their boxes if you new to replace), and put the newer dash into it with the central radio/ switch panel. Gettinng thegearbox out is a massive job, so I'm not surprised he charged a lot!

IIRC the 300 Tdi suffered from timing chain issues if they weren't looked after, so it might be worth looking at although your mileage looks pretty low.