You really need to go to a 6.01 barrel if you have a KWA. Your stock barrel is 6.05. There just honestly is not that much if anything of an improvement between 6.05 and 6.03. When you have an ACM gun and your barrel is 6.1 to 6.18 stock yeah 6.03 is a decent improvement but between 6.05 and 6.03 its really negligible.
http://www.precisionairsoft.com/Classic-Army-G36K-Rail-System-with-Barrel-Set-p/caa227.htm
I have personally installed this kit on a KWA G36C and was happy with how it turned out. The front end is compatible with most brands fore grip so if you can find them they make several different versions but don't always sell them separately. There is the G36 slim handguard, the G36 DMR handguard with attached bipod, the G36K handguard, the railed G36K handguard from above. You have lots of options.
Matt, I'm pretty sure I have that Star G36K kit on my shelf at Precision. It comes with an inner barrel, i want to say it's a 6.04 too. I just checked that site and it's not in our inventory. I'll add it when I get to work so you can look at it.