I think most of your questions have been answered on parts and tuning, there's not a problem in getting parts plus you've got the added advantage of knowing jdm shops in japan
I found my tezza over here so as not having the headache of finding 1 and importing it
there are a lot of auto's over here you have to look a bit harder for a 6mt
you car shouldn't struggle passing the emissions test, but your road tax may cost more due to the test being now being about emissions rather than engine size.
I'm not sure what the law is with importing cars over 10 years old is. I had in my head that they where slightly different from importing newer models
out of interest why is yours a J spec RS