With the limited market for R18 parts I would suggest you look up a local exhaust shop and have a custom system made for you. It sounds scary and I know people like bolt on parts that come in a big box from "super racing parts .com"
Seriously though, the custom route is the way to go especially if you don't want loud and obnoxious. I highly suggest looking into Vibrant components and having a shop build a system around those. A good resonator and muffler can create a nice tone without being ricey. A 2.5" system should work very well on your car. Just be sure the shop has a mandrel pipe bender and doesn't do crush bends.
Vibrant used to make a system for the R18 but it didn't sell well enough so they discontinued it. However, a quick email and they can tell you what parts you need to recreate it. Since the mid pipe is different between coupe/sedan, a bit of custom work is needed anyways. There are a couple R18 systems available already. They are usually quite loud.
