unfortunately, the s2000 is not a car that makes cheap power. if you really want it, get an ls1 or something with a stock turbo.

for sound, try taking the stock air box lid off (free! it takes about 6 seconds to do.). if you like the sound, a FIPK may be worth the money, though the power gains aren't anything amazing.

if you can get $500 or so, look at gears, that will puts more power to the ground at speeds under 100mph. 4.10 is stock. if you go for 4.77 (most aggressive), you get (4.77/4.10)=16% more power at low speeds at the expense of a weaker top end and high rpms while cruising on the highway.

if you can get $1000, you can do NX correctly with a 75 shot (search it), but you'll have to pay (~$50) per bottle. FI is better still, but you have to come up with a lot more money up front--it usually starts around $3k and goes up quickly from there.

 

Removing the airbox lid may make the car sound better, but you actually LOSE power because of the heat soak.