Dont want to put a tangent on your plan but have you looked at this:
$330 + $30 shipping to you for a good high quality 2.5" exhaust CAT Back. with choice of 5 different mufflers.
So $360 out of your $500 budget leaves you $140 for other changes. If you want to stay on budget then I would go with the Trubendz 2.5" flex pipe also for $140... that would put you only around a few dollars over budget. save for the SVT conversion, sell the flex to make up some of the cost.
Or after getting the CAT Back wait, and can check the FS section and see if you can score for an SVT exhaust header, cat and flex for around $250-$300 ish.