In your situation I would definately go for a Disco rather than a Defender. The Defender is an out and out work vehicle - temendously puller, but vey low geared, so poor on long runs. Driver comfort is poor - you have to drive with your knees under your chin and you elbows out the windows.
The Disco will tow - it has the same 3.5 ton towing capacity as the Defender, but is higher geared. Your drive sounds pretyy much like ours, and our Disco does it ok. If we have a full trailer on we need to use the low box, but that is not really an issue. Recommend good tyres though as traction can be a problem - but that would apply to any vehicle. The Td5 has traction control which will avoid wheel spinning. As a family vehicle the disco is really good, comfy and roomy and the TD5 has forward facing rear seats with three point seat belts. A great improvement on the old side facing seats.
I have killed on Tdi, and am just about to trade in the current TDi for a TD5. We only have one car - it will do the farm job, and the family job. Hard to beat I think.