When I got serious about buying my first NSX I followed the fantastic advice the previous posters have said. But, I will add that I first made up my mind as to what years, color and if I wanted a modified or a bone stock NSX. I also had a price range in mind. Then I considered how rare the color options might be. Rare colors take longer to find so be patient if say Grand Prix White is your choice. You are looking at the early 90s models. The first 3 years are all coupes. Though I've graduated to the targa (which I love) I still miss my coupe. The early coupes are amazing in handling and just raw sports car feel. You will find yourself with a smile every time you get behind the wheel. Because of this, I recommend the coupe for a first NSX and also the price is still somewhat affordable but rising. However, I wouldn't get too caught up on price. If you find what you like in the condition you expect, I wouldn't haggle over a couple thousand dollars. Though, that doesn't mean you don't try to get a better deal. Also be ready to pull the trigger when the right car comes up. The really nice ones sell fast.
If you own an S2000, if at all possible keep it. I own both the S2000 and an NSX. I can tell you they are very different cars and fun to drive for different reasons. I had an NSX before I got the S2000 and I was having a hard time passing the NSX in the garage without jumping in for a spin. But, I wanted my NSX to be a weekend car. I needed a fun car for a daily driver or I'd be in the NSX all of the time. So I bought the S2000. It's not too expensive, easy to maintain, reliable and most of all fun to drive. It's like a go-kart. The NSX on the other hand is on a different level. It is an exotic sports car and as such costs a little more to repair and maintain. It will turn heads when you drive down the road due to it's rarity and surprisingly gets a lot of respect from Ferrari/Lambo crowd. Good luck in your search and once you buy that beauty, post some pictures of it so we can all appreciate it.