HK function really well fully repaired. His attack rating was almost as high as my characters. With the superior targeting computer found on the leviathan he will get weapon specialization blaster rifle and pistol and his weapons will do more damager. If you don't want to get him Jamoh Hogra's Carbine or Jurghan Kalta's assault rifle, give him a zabrak tystel Mark III, Zabrak blaster pistol, Madalorian heavy pistol, or a Mandalorian Assault rifle. Zabrak blaster pistol and Mandalorian assault rifle add a chance to stun which helps. Also, don't forget his special weapons such as advanced flamethrower, carbonite projector Mrk III or advanced stun ray. These also make him function really well in your party.
Carth is pretty accurate with his pistols as well I'd say. I pump his dex and give him light armor. Best armor for him is Darth Bandon's fiber armor. Best pistols are Cassus Fett's havy pistol for damager. In his off hand you could give him either a Mandalorian heavy pistol for damage OR something like Saul's sith assasin pistol for stunning capabilities. When facinf shielded opponents such as Saul himself, I usually equip Carth with two Mandalorian Rippers which are disruptors and punch right through Saul's shields. Carth should really be the one to take out Saul anyway
. That way, my character can take out the jedi...
Canderous imo seems better as a melee tank. I up his strength, give him Calo Nord's armor and give him some nice blades. I usually go with two-weapons and give him either poison blades or Ajunta Pall's sword. You could go Bacca's blade, but I think it is over-powered. As a ranged fighter I would get Canderous off heavy weapons because I think they suck. First time I played him I just used his repeater and I felt like he kept missing with it.
Don't forget mission! she has good dex and hits pretty well. I usually pump dex and upgrade rapid shot with her. I then give her a weapon that stuns like Saul's assasin pistol so that she can use her sneak attack. You can also upgrade her dueling which helps her to hit, Throw on the pistol targetting opticsor combat sensor and she is even better.