Over 25 years old, and still basically stock...
No problem, some forms, but easy to do. The DOT has a 25 year cut off and the EPA has a 21 year cut off, so on an older Rover you are all set, no real hassles.

If it is in another country, and not 25 years old or older...
It has to be a certain year and model.
For a list of the years and models that can be imported go here: (They call it "the good list")

http://www.nhtsa.dot.gov/cars/rules/...elig010807.pdf

You can see that only the 1993 Defender 110 and the 1997 Defender 90 are on that list.
You CAN NOT import a 2004 or any other year Defender (unless it is 25 years old).
If the model you want ain't on that list... forget it. It would take an act of Congress and more money than you have to get it on that list.

If it is on that list the next step is that it MUST be imported/ brought up to snuff through a "Registered Importer" and meet all US DOT and EPA requirements. You can't do it yourself, nor can we do it for you.

There is a list of Registered Importers (RIs) here:

http://www.nhtsa.dot.gov/cars/rules/...list010807.htm