LR never sold a diesel Defender in the US. You would have to get a conversion or imported truck. There are people importing Defenders 84 or older and getting the required docs to make them US legal. They come over in all conditions from $1000 vehicles to $60K. There are some diesel Defenders that people have already converted or you could do it yourself. I don't have a Defender, let alone a diesel, and I would imagine support for the diesel won't be as easy as support for a gas version.
Defenders have excellent offroad ability and plenty of upgrades available. The 90 is 2-door, the 110 is 4-door.
A RRC is a good choice or 2004 Disco II or a 2001-2002 P38 on coils, none get good gas mileage though. More offroad support for RRC and Disco.
Hmmm... your thought process is interesting... You say you want an 'expedition' type vehicle, but then your said you want a vehicle that can tackle the Rubicon trail. Usually, these two types of vehicle do not cross-over. Rubicon trail requires lots of clearance, big tires and a short-wheelbase, not the same focus as many expedition-type vehicles. Some do cross-over and Scott Brady did take the Earthroamer XVJP on the Rubicon though...
It all depends on your budget.
Spend some time looking at members build threads to get an idea of what you want.