As i dont have any experience on importing one. However, i have dug deep in the research field to find out that its gonna cost you a lot of money. Hiring a registered importer that has a license to import automobiles. Most of them want a fat down payment to start. Then you have to purchase the car and pay for shipping. Once it gets to cali customs. You will have to pay them to release it to the RI person. Before that, if it doesnt pass cali smog or have the proper windshield and crashbars. You have 90 days to convert it or it gets seized and you are out of the money. However, most RI have access to convert it, so you wont need to worry about that. But you are paying for parts for conversion. But wait, thats not all. Now to get it legally titled amd registered. The paperwork has to be sent to NHTSA...i think thats the name. That could take from 90 days to 18 months before it gets approved. Then once its approved, paperwork and car has to be sent to the state DOT to pass another inspecton. That should be smooth sailing and you can have a street legal JDM car.
Note, research research research. It took me 2-3yrs to gather all this info. It might not take you as long as i did to find this info. I only researched when i felt like finding more info. Oh, one more thing, not every car can be brought into the states. Even though, it might be a 25yr old car, it still has to pass the crash test.
Best option for you. Buy a car that is already federally legal to drive on the streets.