Honestly, the only way you're going to get significant amounts of power is to go with forced induction, and that doesn't come cheap. You may also want to consider 4.10 gears for better acceleration. The gears + install should run about $500-$600.
When it comes to making a V6 Mustang
sound good...well, sound tolerable IMO your best bet is something milder, something along the lines of a Flowmaster 50 series, an H pipe (not X pipe), and shorty headers. Long tubes are overkill here, remember you're only having to deal with 3.8L's of exhaust gasses. IMO going with straight pipes or single chambered mufflers on a V6 is a bad idea. They make your car sound like a farming implement.