Thrill, its best to find a 400i that's already in the U.S. It'll more than likely have the D.O.T./E.P.A. paperwork to ASSIST in registering the car. You must familiarize yourself with your local or state laws concerning these grey-market cars. Most areas have emissions-testing exemptions based on vehicle age (which I'm not 100% sure if the 400 would qualify. Most exemptions are for cars 25 years old and older. Again, check your area for more info).
Your choice of a fuel injected car will make life much easier for you. They're naturally more efficient and pollute less than a carb car, but considering its a near-five liter V-12, its still dumping loads of HC into the air... Installing temporary catalytic converters is one way to pass (albeit not too legal if its temporary...). Its best to sort this out with the vehicle seller and a mechanic.
Although this is way before my time of ownership, my own 308 GT4 was serviced and offered for sale at Foreign Cars! Of course I can't comment on their work as this happened back in 1981-83, but from what I've heard from other Ferrari owners, they are a good place.
Yes, the easiest choice is the 308/328/Mondial. All are smog legal, there are plenty of them out there so they're reasonable in price. And they're easy to own and service. No real difference in service cost between the V-8 and V-12. The V-8's require more service, but its less costly per, the V-12's require less service, but more expensive per. A rough guide to ENGINE REBUILD COSTS: $1000/cylinder - and that's if nothing is broken. ROUTINE SERVICE (filters, fluids, adjustments, etc...): $600-$1000 total. Again, these are just examples, it all depends on the car you're looking at...