Dyno tune is not highly recommended, it is required or you WILL grenade your car. $3500 for that used kit... if that includes all the fuel stuff and every other little thing that usually nickels/dimes to death that is a good deal.
Honestly though, for a high schooler with a budget I would not supercharge it. The numbers you are already making out of that 3V are really good for that year. Also, what sort of suspension/wheel/tire setup do you have? If you are mostly stock suspension all you are going to do is spin your tires with a supercharger and probably are doing that now if you are on stockish suspension at 320rwhp.
-Good aftermarket UCA and LCAs
-Adjustable PHB or (better) the Whiteline or upcoming BMR Watt's link
-Good set of wheels/tires
Much MUCH better to do all that than a supercharger.
2000 Mustang GT Steeda build #48. Bolt ons, suspension, Novi 2000.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.