First, I would like to welcome you to the forum. I am pretty sure you will learn a lot about NSX here. As for your questions....let me try to give it a shot.
1) You sould have no problems running 17 and 18's. However, if you want to go with this setup, make sure you use lower profile tires, else you might expereince some "rubbing" of the tires against your fenders. I used to run 18x7 and 19x10, but that was mostly for aesthetics only.
2)You won't see a "sudden" gain in hp and torque with mods you mentioned. If I were you, I would just get the exhaust and intake....headers are not really necessary unless you want to go wtih supercharger.
3)I think you are talking about NSXmodified....yeah Larry (the owner) does not really answer the phone. It might be best to swing by his shop...if you need to directions let me know.
4) Good luck with your search....I am sure you will find the right car.
If you live in Orange County, you are more than welcome to come to the monthly meetings at NSX modified...usually there are about 10-15 cars that show up, and it is a great time to ask your questions. Trust me, all of the Sol.Cal onwers are friendly and helpful. I can also take you for a ride on my single turbo setup...it is pretty fun......Hope this helps.