The entry level Decimator isn't a bad buy. A number of builds are effective with it and Darth Vader as a crew member allows for higher pilot skill wingmen to remove the shields from targets before using his ability. If you're up against extremely agile ships, Gunner not only increases the odds in your favour, but also allows Vader to trigger twice.
Rebel Captive is always a good way to deter people from shooting at the Decimator and having zero agility, it needs all the help it can get. Ysanne Isard is a good choice, but fits better with Commander Kenkirk.