Probably the best place to start is the National Highway Transportation Safety Administration (NHTSA) web site at http://www.nhtsa.gov/cars/rules/import/.
There you'll find a wealth of info including Registered Importers who you'll need to do the necessary conversions and paperwork. You'll also see a link to the vehicle eligibility list which shows that currently only the '93 D110 and '94-98 Disco are eligible for import. The Disco is a new addition I saw listed for the first time while writing this reply! As for shipping any other year models of the D110, they will first need to be on this eligibility list. Now you're probably asking, how does this happen? Well its stated in the header of the list,
"VCP eligibility numbers are assigned to vehicles that are decided to be eligible under 49 U.S.C. § 30141(a)(1)(B), based on a petition from a manufacturer or registered importer which establishes that the vehicle has safety features that comply with, or are capable of being altered to comply with, all applicable Federal motor vehicle safety standards. Vehicles for which eligibility decisions have been made are listed below alphabetically by make. Eligible models within each make are listed numerically by "VSA," "VSP," or "VCP" number."
As you can see then someone has petitioned the government to allow the '93 D110 (VSP #212) and '94-98 Discos (VSP # 338) to be imported. I suspect that the '93 D110 was the easiest year to justify since they were sold in the US that year. I'd be willing to bet its possible to have a similar petition approved for a later model if someone would be willing to do the leg work and push the petition. I talked to the folks at US DOT and EPA offices in Washington DC while going though my research and they said the petition was about $500. Unfortunately, the petition is not guaranteed to be approved and the $500 is not refundable. The guy I talked to was George Entwistle and I believe his number is (202) 927-0771 (ext 5306 or 5308), or try (800) 424-9393. Another name I found in my notes is Bob DeSoucey at (212) 637-7035. I believe he is a government motor import specialist. I don't know the particulars of what's required in the petition. You'll have to check with the experts on that one.
Take care of everything including shipping cost and posting all bonds etc. This could add a few thousand to your overhead cost if you have to take care of everything yourself. Also, it may be tougher to bring a vehicle into the west coast due to California emission standards, etc. All things to consider.
As for the questions on the diesel vs gas engine, I believe you can not go wrong with the diesel powerplant for a die-hard off road machine. There are definitely more diesels than gas versions in the UK and I think the Brits have it figured out pretty well. When you compare the low end torque of the diesel to the power of a gas engine, I don't think the power gives you any advantage in the sticky stuff unless you are trying to pull huge loads. The low end torque allows you to move without excessive wheel spin which will stop any vehicle in its tracks. I experienced this about two weeks ago when I was pulling my friends RR out of thick sand. The fuel economy is a definite plus as well. My 200Tdi gets about 27-30mpg. Although, I don't expect to ever peel out from a stand still.