maybe gain 15-20 hp.

Go to a muffler shop or place that does exhausts. Get a quote and tell them the kind of power you want. Low, mid, high. The muffler sizes can change where you get the most out of it.

You can try picking up certain parts at different places (ie online, mailorder) for better prices, bring it all to a muffler place and have them just do the rest. like the turbo and downpipe, there are different grades of steel for the downpipe and bending techniques that will give you more power.

Could run you $300 for simple mod to it, full would be more.