If you're going with an Arc Hammer to start on your Arms Master (which I strongly endorse and love), maybe consider swapping the Combat Shotgun for a regular Shotgun? If they're within 12" or 8", you might be looking to charge and smash rather than shoot. You could use the saved credits to put a Flail on that second Wrecker and upgrade both Wreckers to Autopistols.
Alternately, hold what you have and make those upgrades a priority. A Combat Shotgun is never a bad choice.