Since you want rumble (as opposed to something engineered & tested for documented power =$$$$), then by far your most economical approach is to decide on level of sound and tone you're after and have a local shop fab exhaust using mufflers of your preference. They can replace rear section (thus allowing you to keep original parts if you ever want to put it back to stock). Or as much or little as you want. Personally I'd at least keep the original mufflers for future replacement when time to replace the car. Kits designed for the car are, for some reason, priced somewhere twixt crazy and "Dude, are you on glue?"

Original system IS stainless steel, as are a number of pre-designed aftermarket systems, so if that's a route you want to take, rock on and go for it.