The pre 1967 vehicles will not need any emissions clearance because they precede the first of the US emissions equirements. Even if the vehicle is 25 years or older, it still is subject to Arizona's emissions tests, starting with the 1967 requirements. AZ operates outside of the 25 year federal exemptions. 25 years old will get you into the US, but no guarantee of a title or registration in AZ, that's were it gets sticky in our state.
The challenge in finding a pre 67 vehicle will be to find one that's still sound, considering the damp environment over there. If you find something, make sure it's passed a recent MOT test. As Ian can testify, the MOT tests are exhaustive and all inclusive. If it passes that, you should have a pretty sound vehicle. It would also help to have another Rover owner take a look at it while it's there to see if it's a good value.
The URL that Lou sent has had both good and bad reviews from people that have used them in the past. A common tactic is to put a fresh coat of paint on them and ship them overseas. Many of the problem vehicles bought buy US owners wouldn't have passed a UK MOT inspection. In many cases the problems with frames, bulkheads and body panels start to surface within six months after arrival ... about the time for the cosmetic patches to wear thin. Be very careful with these guys. There is no recourse in case of a problem, you and your problems are here, while they're over there with your money. Not everyone has had problems here, but many have been very vocal about this place on the LRO list over the past several years. Their RO-RO prices seam a little high too.
Something you may try is to e-mail Frank Elson, he writes for LRM (e-mail address can be found within the magazine). He's said in the past that he can help locate a sound vehicle for prospective owners on the market. Since he's in the UK, the available vehicles will probably be RHD.
To be honest, if you can find a suitable vehicle already over here, you'll avoid a lot of hassles and even more red tape. My 109 came from three guys in Washington state that were going to set the world on fire by importing containers full of Rovers from the UK. By the time they got one container over here with two vehicles and one via RO-RO (Roll On - Roll Off) they were glad to dump them. They said that every time they turned around, someone was trying to nickel and dime them, on both sides of the pond. It can be done, but you have to be very careful with the details, both in the vehicle selection and the importing.
Several months ago, a guy contacted me with a 109 for sale, three door regular RHD SIII, with no title. How he actually got it off the boat, I have no idea. Unless he plays the title / VIN switching game, it going to be broken down for parts. Since it was a RHD vehicle, it's going to raise red flags every time it tries to get the VIN verified (physical inspection) for a title inspection. Apparently, that's why he was shopping it to out of state prospects, he's exhausted all of his local options. Getting it insured with bogus numbers can be a problem too. Don't misunderstand, it is fairly easy to do with a Series truck, but you need a legit title to "swap" to in the first place. You can buy blank VIN plates from BP and stamp the corresponding numbers on it to match the title. But, you didn't hear that from me.
Always remember, you're buying a used car. I can't think of a subject with more caveats attached to it. When you find a good one, you'll love it ... it's the journey to your destination that will be the real challenge.